Noon service? It's on time (4)
I believe the answer is:
sext
'time' is the definition.
(sext is a kind of time)
'noon service? it's' is the wordplay.
I cannot quite see how this works, but
'service?' could be 'sex' (service is a kind of sex) and 'sex' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'on' acts as a link.
Can you help me to learn more?
(Other definitions for sext that I've seen before include "service" , "Explicit message" .)