Bond is to become smaller (8)
I believe the answer is:
contract
This clue is a double definition.
'bond' is the first definition.
(thesaurus)
'become smaller' is the second definition.
(I've seen this before)
'is to' is the link.
(Other definitions for contract that I've seen before include "four spades, perhaps" , "Binding agreement between two or more" , "Shrink in legal agreement?" , "Shrink in written agreement" , "Become smaller" .)