Join up, never imagining terrorist enemies initially (5)
I believe the answer is:
unite
'join' is the definition.
(synonyms)
'up never imagining terrorist enemies initially' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'up never imagining terrorist enemies' is 'UNITE'.
(Other definitions for unite that I've seen before include "Join together, become one" , "old coin" , "Couple" , "Link" , "Become an integrated whole" .)