Swinger frolicking nude in fruit (8)
I believe the answer is:
pendulum
'swinger' is the definition.
(something which swings)
'frolicking nude in fruit' is the wordplay.
'frolicking' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'fruit' becomes 'plum' (plum is a kind of fruit).
'nude' is an anagram of 'endu'.
'endu' going inside 'plum' is 'PENDULUM'.
(Other definitions for pendulum that I've seen before include "Vacillator" , "Clock regulator" , "Moves to and fro" , "Oscillating mechanism" , "Lead swinging" .)