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