A number of flutters, then a nap (5,5)
I believe the answer is:
forty winks
'a nap' is the definition.
(synonyms)
'a number of flutters' is the wordplay.
'a number' becomes 'forty' (forty is a kind of number).
'of flutters' becomes 'winks' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'forty'+'winks'='FORTY WINKS'
'then' acts as a link.
Can you help me to learn more?
(Other definitions for forty winks that I've seen before include "Quick nap" , "Kip" , "Rest" , "Snooze" .)