First-class soldier thanks heartless type for trouble (7)
I believe the answer is:
agitate
'for trouble' is the definition.
(agitating is a kind of troubling)
'first-class soldier thanks heartless type' is the wordplay.
'first class' becomes 'A' (as in A-list, A-team etc).
'soldier' becomes 'GI' (term for US soldier).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'heartless' means to remove the middle letters.
'type' with its middle taken out is 'te'.
'a'+'gi'+'ta'+'te'='AGITATE'
(Other definitions for agitate that I've seen before include "Stir (up public opinion?)" , "'Stir, excite (7)'" , "Disturb or stir up" , "Stir violently" , "Trouble, make nervous" .)