Join military group heading for Edinburgh (5)
I believe the answer is:
unite
'join' is the definition.
(I know that join can be written as unite)
'military group heading for edinburgh' is the wordplay.
'military group' becomes 'unit' (group of soldiers).
'heading for' suggests taking the first letters.
The first letter of 'edinburgh' is 'e'.
'unit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "Pull together" , "ally" , "old coin" , "Come, or join together" , "Make one" .)