Dimmest female in a tricky exam (8)
I believe the answer is:
faintest
'dimmest' is the definition.
('faint' can be a synonym of 'dim')
'female in a tricky exam' is the wordplay.
'female' becomes 'f' (common abbreviation).
'tricky' indicates an anagram.
'exam' becomes 'test' (synonyms).
'in'+'a'='ina'
'ina' with letters rearranged gives 'ain'.
'f'+'ain'+'test'='FAINTEST'
(Other definitions for faintest that I've seen before include "most dim" , "Most unenthusiastic" .)