Decline to have party wearing dress (2,4)
I believe the answer is:
go down
'decline' is the definition.
(I know that decline can be written as go down)
'party wearing dress' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'wearing' means one lot of letters goes inside another.
'dress' becomes 'gown' (gown is a kind of dress).
'do' inserted within 'gown' is 'GO-DOWN'.
'to have' acts as a link.
(Other definitions for go down that I've seen before include "Fall" , "Decline" , "Subside" , "lose to" , "Descend - be relegated" .)