Hollow blocks harden and contract (9)
I believe the answer is:
indenture
'contract' is the definition.
(indenture is a kind of contract)
'hollow blocks harden' is the wordplay.
'hollow' becomes 'dent' (I've seen this before**).
'blocks' means one lot of letters goes inside another (the inside letters form a 'blockage').
'harden' becomes 'inure' ('inure' can be a synonym of 'harden').
'dent' inserted inside 'inure' is 'INDENTURE'.
'and' acts as a link.
(Other definitions for indenture that I've seen before include "Agreement binding apprentice to a master" , "contract" , "Document" , "Sealed agreement" , "served by a novice?" .)