Infatuated, set to frolicking in bed (8)
I believe the answer is:
besotted
'infatuated' is the definition.
(I've seen this before)
'set to frolicking in bed' is the wordplay.
'frolicking' indicates an anagram.
'in' means one lot of letters goes inside another.
'set'+'to'='setto'
'setto' anagrammed gives 'sotte'.
'sotte' placed within 'bed' is 'BESOTTED'.
(Other definitions for besotted that I've seen before include "Stupefied by infatuation" , "Showing foolish fondness" , "Infatuated - so betted accordingly" , "much taken" , "Bested to become infatuated" .)