Forceful six are annoying (6)
I believe the answer is:
virile
'forceful' is the definition.
(I've seen this in another clue)
'six are annoying' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'are annoying' becomes 'rile' (to rile someone is to annoy them).
'vi'+'rile'='VIRILE'
(Other definitions for virile that I've seen before include "Robustly masculine" , "male" , "Having energy" , "Potent" , "Masculine and macho" .)