Join military group with energy (5)
I believe the answer is:
unite
'join' is the definition.
('unite' can be a synonym of 'join')
'military group with energy' is the wordplay.
'military group' becomes 'unit' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'unit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "ally" , "Wed?" , "Pull together" , "Join together, become allies" , "Come, or join together" .)