Aggressively masculine yet essentially fetching (5)
I believe the answer is:
butch
'aggressively masculine' is the definition.
(I've seen this before)
'yet essentially fetching' is the wordplay.
'yet' becomes 'but' (synonyms).
'essentially' means to look at the middle letters.
The central letters of 'fetching' are 'ch'.
'but'+'ch'='BUTCH'
(Other definitions for butch that I've seen before include "Acting as a male (colloq.)" , "Aggressively masculine" , "Macho" , "Tough" , "Markedly masculine" .)