Balloon, first of many captured by radar image (5)
I believe the answer is:
blimp
'balloon' is the definition.
(thesaurus)
'first of many captured by radar image' is the wordplay.
'first of' says to take the initial letters.
'captured by' is an insertion indicator.
'radar image' becomes 'blip' (I've seen this before).
The initial letter of 'many' is 'm'.
'm' going into 'blip' is 'BLIMP'.
(Other definitions for blimp that I've seen before include "returns one pound to provost" , "Small nonrigid airship" , "Pompous type - barrage balloon" , "full of hot air?" , "aircraft" .)