Custom doesn't regularly last (6)
I believe the answer is:
utmost
'last' is the definition.
('utmost' can be a synonym of 'last')
'custom doesn't regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'custom'+'doesnt'='customdoesnt'
The alternating letters of 'customdoesnt' are 'UTMOST'.
(Other definitions for utmost that I've seen before include "Highest in degree" , "Paramount" , "(To the) maximum extent" , "The greatest possible degree" , "Extremest" .)