Regularly enter an exam: that’s most clever (7)
I believe the answer is:
neatest
'most clever' is the definition.
(I've seen this in another clue)
'regularly enter an exam' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'exam' becomes 'test' (synonyms).
'enter'+'an'='enteran'
The alternate letters of 'enteran' are 'nea'.
'nea'+'test'='NEATEST'
'that's' is the link.
(Other definitions for neatest that I've seen before include "Most smart" , "Most orderly" , "best-looking desk, say?" , "Most tidy" , "Seat ten (anag)" .)