Make one military group start to exercise (5)
I believe the answer is:
unite
'make one' is the definition.
(I've seen this before)
'military group start to exercise' is the wordplay.
'military group' becomes 'unit' (group of soldiers).
'start to' suggests taking the first letters.
The first letter of 'exercise' is 'e'.
'unit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "Join together, merge" , "Amalgamate" , "fuse" , "Group of workers" , "Combine - join" .)