Those who play naturally don't do this (7)
I believe the answer is:
overact
'those' is the definition.
I can't tell whether this defines the answer.
'play naturally don't do this' is the wordplay.
I cannot really see how this works, but
'play' could be 'over' (over is a kind of play) and 'over' is found in the answer.
'do' could be 'act' (I've seen this before) and 'act' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'who' is the link.
Can you help me to learn more?
(Other definitions for overact that I've seen before include "pile it on" , "Do what ham thespian does" , "Perform hammily" , "Perform in an exaggerated manner" , "Play part exaggeratedly" .)