Sound the time - lunchtime (4)
I believe the answer is:
tone
'sound' is the definition.
(I know that tone is a type of sound)
'the time lunchtime' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'time' could be 'one' (I've seen this before) and 'one' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tone that I've seen before include "Colour quality; note" , "Musical sound; quality of hue" , "Sound - style" , "Bodily firmness" , "A pitch of note" .)