Claim is not commonly seen in the main (8)
I believe the answer is:
maintain
'claim' is the definition.
(to maintain is to continue to claim something)
'not commonly seen in the main' is the wordplay.
'not commonly' becomes 'aint' (I've seen this in other clues).
'seen in' indicates putting letters inside.
'aint' inserted inside 'main' is 'MAINTAIN'.
'is' is the link.
(Other definitions for maintain that I've seen before include "Continue with an opinion" , "Conserve" , "Uphold, preserve" , "Keep in existence or in repair" , "declare" .)