Firm defends working this evening (7)
I believe the answer is:
tonight
'this evening' is the definition.
(synonyms)
'firm defends working' is the wordplay.
'firm' becomes 'tight' (synonyms).
'defends' is an insertion indicator (inserted letters are held or defended).
'working' becomes 'on' (on can mean working or activated).
'tight' enclosing 'on' is 'TONIGHT'.
(Other definitions for tonight that I've seen before include ""Not ____, Josephine!"" , "after supper?" , "Hotting up for after sundown" , "Late today" , "now or very soon" .)