This evening firm will restrict working! (7)
I believe the answer is:
tonight
'this evening' is the definition.
(synonyms)
'firm will restrict working' is the wordplay.
I cannot quite see how this works, but
'working' could be 'on' (on can mean working or activated) and 'on' is located in the answer.
'firm' could be 'tight' (synonyms) and 'tight' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tonight that I've seen before include "Late today" , "Got thin (anag)" , "now or very soon" , "Later today" , "Hotting up for after sundown" .)