Stay for start of party (4)
I believe the answer is:
prop
'stay' is the definition.
(I've seen this before)
'for start of party' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'start of' says to take the initial letters.
The initial letter of 'party' is 'p'.
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "one playing Rugby" , "Lean" , "Support to keep from falling" , "Front row forward" , "needed at certain stages" .)