Mock exam occasionally set that’s tricky (5)
I believe the answer is:
tease
'mock' is the definition.
(teasing is a kind of mocking)
'exam occasionally set that's tricky' is the wordplay.
'occasionally' means one should take alternating letters.
'that's tricky' indicates an anagram.
The alternate letters of 'exam' are 'ea'.
'ea'+'set'='easet'
'easet' anagrammed gives 'TEASE'.
(Other definitions for tease that I've seen before include "Banter, make gentle fun of" , "Gently mock" , "Taunt or bait" , "Twit - flirt" , "Deride" .)