Composer close to Bristol is captured by army (5)
I believe the answer is:
holst
'composer' is the definition.
(Gustav Holst)
'close to bristol is captured by army' is the wordplay.
'close to' says to take the final letters (the 'close' to the word).
'is captured by' is an insertion indicator.
'army' becomes 'host' (host is a kind of army).
The last letter of 'bristol' is 'l'.
'l' put into 'host' is 'HOLST'.
(Other definitions for holst that I've seen before include "Scorer" , "Gustav --, English composer" , "musical writer" , "Composer of The Planets, d. 1934" , "Planets Suite composer" .)