Movement: a good one to gatecrash a party (6)
I believe the answer is:
adagio
'movement' is the definition.
(type of musical movement)
'a good one to gatecrash a party' is the wordplay.
'a good' becomes 'ag' (I've seen this in other clues**).
'one' becomes 'i' (Roman numeral).
'to gatecrash' means one lot of letters goes inside another.
'party' becomes 'do' (do is a kind of party).
'ag'+'i'='agi'
'a'+'do'='ado'
'agi' going into 'ado' is 'ADAGIO'.
(Other definitions for adagio that I've seen before include "Slowly and graceful, in music" , "In slow tempo" , "moving slowly" , "In music, leisurely" , "Slowly (mus.)" .)