Legally bound to have shrunk (10)
I believe the answer is:
contracted
This clue might be a double definition.
'legally bound' is the first definition.
'contracted' can be an answer for 'bound' (thesaurus). I'm not sure about the 'legally' bit.
'shrunk' is the second definition.
(contracting is a kind of shrinking)
'to have' acts as a link.
Can you help me to learn more?
(Other definitions for contracted that I've seen before include "Got shorter" , "Formally agreed; shrank" .)