Slowly in trouble suppressing a grunt (6)
I believe the answer is:
adagio
'slowly' is the definition.
(musical term)
'trouble suppressing a grunt' is the wordplay.
'trouble' becomes 'ado' (both can mean bother or commotion).
'suppressing' is an insertion indicator.
'grunt' becomes 'gi' (both can mean a US soldier).
'a'+'gi'='agi'
'ado' placed around 'agi' is 'ADAGIO'.
'in' is the link.
(Other definitions for adagio that I've seen before include "Slow and graceful, musically" , "Leisurely tempo" , "Musical composition to be played slowly and gracefully" , "develops slowly" , "Slowly and gracefully, musically" .)