Pay for entertaining day (4)
I believe the answer is:
fund
'pay for' is the definition.
(synonyms)
'entertaining day' is the wordplay.
'entertaining' becomes 'fun' (similar in meaning).
'day' becomes 'd' (abbreviation for day).
'fun'+'d'='FUND'
(Other definitions for fund that I've seen before include "Reserve of money set aside for a purpose" , "Supply cash" , "Finance" , "Provide with money, say project" , "Sponsor" .)