Pleased to include rental agreement (5)
I believe the answer is:
lease
'rental agreement' is the definition.
(eg rental agreement for a flat)
'pleased to include' is the wordplay.
'to include' indicates a hidden word.
'LEASE' is hidden in the letters of 'pleased'.
(Other definitions for lease that I've seen before include "Letting contract" , "Contract for, say, use of building" , "Contract for use of land or building" , "Rent (a property)" , "Contract of tenure" .)