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' (GI can mean a US soldier).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'heartless' means to remove the middle letters.
'type' with its centre 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" .)