Firm about working later this evening (7)
I believe the answer is:
tonight
'later this evening' is the definition.
(I've seen this before)
'firm about working' is the wordplay.
'firm' becomes 'tight' (synonyms).
'about' is an insertion indicator.
'working' becomes 'on' (on can mean working or activated).
'tight' enclosing 'on' is 'TONIGHT'.
(Other definitions for tonight that I've seen before include "Hotting up for after sundown" , "later in the day" , "Hotting (anag.)" , ""Not ____, Josephine!"" , "Later today" .)