Certainly not Falstaff for Thursday evening perhaps (9)
I believe the answer is:
weeknight
'certainly not falstaff for thursday' is the definition.
I can't judge whether this defines the answer.
'evening perhaps' is the wordplay.
I cannot really understand how this works, but
'evening' could be 'night' (I've seen this in another clue) and 'night' is located in the answer.
The remaining letters 'week' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for weeknight that I've seen before include "evening between Monday and Friday" , "Saturday evening is not" , "will be wanting an early bedtime then?" , "time to sleep after work" , "End of working day" .)