Join some troops heading east (5)
I believe the answer is:
unite
'join' is the definition.
(I know that join can be written as unite)
'some troops heading east' is the wordplay.
'some troops' becomes 'unit' (I've seen this in another clue).
'heading east' becomes 'e' (abbreviation).
'unit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "Join forces" , "Group of workers" , "Come together as allies or comrades" , "Amalgamate" , "Join together, merge" .)