Fighter once assembled eastern troops (6)
I believe the answer is:
meteor
'fighter once' is the definition.
(former British fighter jet)
'assembled eastern troops' is the wordplay.
'assembled' becomes 'met' ('meet' can be a synonym of 'assemble').
'eastern' becomes 'e'.
'troops' becomes 'or' (military abbreviation for other ranks).
'met'+'e'+'or'='METEOR'
(Other definitions for meteor that I've seen before include "Glowing heavenly body" , "lump of rock" , "Shooting star" , "Body entering earth's atmosphere" , "Body falling from space" .)