Gents: HRT is fantastic for potency! (8)
I believe the answer is:
strength
'for potency' is the definition.
(synonyms)
'gents hrt is fantastic' is the wordplay.
'is fantastic' is an anagram indicator (a fantastic or improbable ordering of the letters).
'gents'+'hrt'='gentshrt'
'gentshrt' is an anagram of 'STRENGTH'.
(Other definitions for strength that I've seen before include "Forcefulness" , "Vigour, potency" , "Potency - firmness" , "Physical might, muscle" , "Concentration" .)