Deal involves a constraint (11)
I believe the answer is:
transaction
'deal' is the definition.
(I've seen this before)
'involves a constraint' is the wordplay.
'involves' indicates an anagram.
'a'+'constraint'='aconstraint'
'aconstraint' is an anagram of 'TRANSACTION'.
(Other definitions for transaction that I've seen before include "Piece of business" , "Act of buying/selling" , "Instance of buying/selling" , "Deal, negotiation" .)