Slow movement gets a soldier in trouble (6)
I believe the answer is:
adagio
'slow movement' is the definition.
(slow musical movement)
'a soldier in trouble' is the wordplay.
'soldier' becomes 'GI' (term for US soldier).
'in' indicates putting letters inside.
'trouble' becomes 'ado' (both can mean bother or commotion).
'a'+'gi'='agi'
'agi' placed within 'ado' is 'ADAGIO'.
'gets' is the link.
(Other definitions for adagio that I've seen before include "Slow musical piece or movement" , "Leisurely, musically speaking" , "Musically, in slow time" , "Musical composition to be played slowly and gracefully" , "leisurely movement" .)