Deployed to the war, causing trouble (3,5)
I believe the answer is:
hot water
'trouble' is the definition.
(I've seen this before)
'deployed to the war' is the wordplay.
'deployed' is an anagram indicator.
'to'+'the'+'war'='tothewar'
'tothewar' anagrammed gives 'HOT WATER'.
'causing' is the link.
(Other definitions for hot water that I've seen before include "that comes at bath time?" , "(Wet?) trouble" , "A state of trouble" , "Bed warmer" , "The naughty get in it" .)