Some grumble as extending contract (5)
I believe the answer is:
lease
'contract' is the definition.
(leasing is a kind of contracting)
'some grumble as extending' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'LEASE' can be found hidden inside 'grumble as extending'.
(Other definitions for lease that I've seen before include "Tenancy contract" , "Contract for use of land or building" , "Contract for, say, use of building" , "Property rental agreement" , "Charter" .)