Unhappy to be missing every second of forty winks (3)
I believe the answer is:
nap
'forty winks' is the definition.
(synonyms)
'unhappy to be missing every second' is the wordplay.
'to be missing every second' indicates alternate letters.
The alternate letters of 'unhappy' are 'NAP'.
'of' is the link.
(Other definitions for nap that I've seen before include "A brief informal sleep" , "A short sleep or snooze" , "One likely to win" , "soft, fluffy surface" , "Short raised fibres on cloth on snooker table" .)