Join military group to east (5)
I believe the answer is:
unite
'join' is the definition.
('unite' can be a synonym of 'join')
'military group to east' is the wordplay.
'military group' becomes 'unit' (I've seen this before).
'to east' becomes 'e' ('E' can be a synonym of 'east').
'unit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "Join together, merge" , "Come, or join together" , "Amalgamate" , "ally" , "Connect" .)