Couple starts to play around in relationship (4)
I believe the answer is:
pair
'couple' is the definition.
('pair' can be a synonym of 'couple')
'starts to play around in relationship' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'play around in relationship' is 'PAIR'.
(Other definitions for pair that I've seen before include "Duo" , "players of duet?" , "Two of a similar type" , "Twosome" , "Agreement to avoid division" .)