Play day in pool (4)
I believe the answer is:
fund
'pool' is the definition.
(thesaurus)
'play day' is the wordplay.
'play' becomes 'fun' ('fun' can be a synonym of 'play'**).
'day' becomes 'd'.
'fun'+'d'='FUND'
'in' is the link.
(Other definitions for fund that I've seen before include "Stock, resources" , "Pool of money" , "Finance" , "Sum of money set aside for a purpose" , "Stock of money available for particular purpose" .)