Army brag derisively at first (5)
I believe the answer is:
crowd
'army' is the definition.
(I've seen this before)
'brag derisively at first' is the wordplay.
'brag' becomes 'crow' (crowing is a kind of bragging).
'at first' suggests taking the first letters.
The initial letter of 'derisively' is 'd'.
'crow'+'d'='CROWD'
(Other definitions for crowd that I've seen before include "Large number of people gathered together" , "Large number of people in one place" , "Large group of people" , "Throng" , "Mob of people" .)