Monday evening, say, was insignificant, sir? (9)
I believe the answer is:
weeknight
'monday evening' is the definition.
(I've seen this before)
'insignificant sir?' is the wordplay.
'insignificant' becomes 'wee'.
'sir?' becomes 'knight' (knights have the title 'sir').
'wee'+'knight'='WEEKNIGHT'
'say was' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for weeknight that I've seen before include "appears between working days" , "Saturday evening is not" , "time to sleep after work" , "Bit of time" , "evening between Monday and Friday" .)