Slow movement of a soldier gatecrashing a party (6)
I believe the answer is:
adagio
'slow movement' is the definition.
(slow musical movement)
'a soldier gatecrashing a party' is the wordplay.
'soldier' becomes 'GI' (term for US soldier).
'gatecrashing' indicates putting letters inside.
'party' becomes 'do' (do can informally mean a party).
'a'+'gi'='agi'
'a'+'do'='ado'
'agi' placed into 'ado' is 'ADAGIO'.
'of' acts as a link.
(Other definitions for adagio that I've seen before include "In slow tempo" , "In slow time (music)" , "(A movement) to be played slowly" , "'Slowly and gracefully, in music (6)'" , "Slowly (mus.)" .)