Claim mastery, after working: that’s irregular (12)
I believe the answer is:
asymmetrical
'irregular' is the definition.
('asymmetrical' can be similar in meaning to 'irregular')
'claim mastery after working' is the wordplay.
'after working' indicates an anagram.
'claim'+'mastery'='claimmastery'
'claimmastery' is an anagram of 'ASYMMETRICAL'.
'that's' acts as a link.
(Other definitions for asymmetrical that I've seen before include "a gymnast would use such bars" , "Crooked" , "Not even" , "Irregular in outline" .)