Contract ends once broken (8)
I believe the answer is:
condense
'contract' is the definition.
(synonyms)
'ends once broken' is the wordplay.
'broken' is an anagram indicator.
'ends'+'once'='endsonce'
'endsonce' with letters rearranged gives 'CONDENSE'.
(Other definitions for condense that I've seen before include "contract?" , "Reduce volume" , "Turn from gas to liquid" , "Decrease, reduce" , "Send once (anag.)" .)