Badge one gains in manoeuvres (8)
I believe the answer is:
insignia
'badge' is the definition.
(insignia is a kind of badge)
'one gains in manoeuvres' is the wordplay.
'one' becomes 'i' (Roman numeral).
'manoeuvres' indicates anagramming the letters.
'gains'+'in'='gainsin'
'gainsin' anagrammed gives 'nsignia'.
'i'+'nsignia'='INSIGNIA'
(Other definitions for insignia that I've seen before include "Badges of office" , "I sign in a funny way to get badges or emblems" , "Emblems of rank" , "arms" , "Stars and stripes, say" .)