Let fifty relax (5)
I believe the answer is:
lease
'let' is the definition.
(as in to lease or let a building)
'fifty relax' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'relax' becomes 'ease' (eg to ease a restriction is to relax it).
'l'+'ease'='LEASE'
(Other definitions for lease that I've seen before include "Hire out, rent" , "Contract to rent" , "Rent -- easel (anag)" , "Property rental agreement" , "Term of a rental contract" .)