Clarity is not a ground for opposition. Opposition proceedings are not designed as a procedure for generally amending (or revoking) patents that contain any kind of defect, and therefore opposition proceedings are not to be regarded as a continuation of examination proceedings. As a general rule this means that a granted claim has to be lived with even if new facts (e.g. new prior art) demonstrate that the claim is unclear (G 3/14).
In considering whether, for the purpose of Art. 101(3), a patent as amended meets the requirements of the EPC, the claims of the patent may be examined for compliance with the requirements of Art. 84 only when, and then only to the extent that, an amendment introduces non-compliance with Art. 84 (G 3/14, confirming the jurisprudence as exemplified by T 301/87). A lack of compliance with Art. 84 cannot be seen as having been introduced by an amendment if a clarity problem already present in the claims as granted is only brought into notice, highlighted or made visible by the amendment.
According to G 3/14, the amendment of one claim or part of a patent cannot lead to a re-examination of other parts of the patent which have not been amended. Thus, the deletion of an independent claim with its dependent claims or the deletion of a dependent claim leaving the independent claims and other dependent claims intact does not permit examination of the remaining claims for compliance with Art. 84.
A claim amended during opposition proceedings is not subject to examination for compliance with Art. 84 if it results from
(iii)deleting wording from a granted claim (whether independent or dependent), whereby its scope is narrowed but a pre-existing lack of compliance with Art. 84 is left intact (as exemplified by T 301/87); or
However, an amended claim is to be examined for compliance with Art. 84:
(vi)if a feature from a dependent claim as granted is introduced into an independent claim as granted and this feature was previously connected with other features of that dependent claim and an alleged lack of compliance with Art. 84 is introduced by the amendment.