Sat maybe on call, peak time (6)
I believe the answer is:
'sat' is the definition.
The answer and definition can be both related to time as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'maybe on call peak time' is the wordplay.
'maybe on call peak' becomes 'hey' (I can't justify this - if you can you should believe this answer much more).
'time' becomes 'day' (day is a kind of time).
Can you help me to learn more?
(Other definitions for heyday that I've seen before include "One's peak period" , "Period of one's greatest success" , "The flush of vigour, health etc." , "Successful period" , "Time of greatest popularity" .)