Stay for start of play (4)
I believe the answer is:
prop
'stay' is the definition.
(I've seen this before)
'for start of play' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'start of' suggests taking the first letters.
The first letter of 'play' is 'p'.
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "A rigid support - 37 Across is one" , "What actor may carry" , "Lean" , "Support (a rugby player?)" , "Object used on the set of a play" .)