Piece of gym equipment making one smile (4)
I believe the answer is:
beam
This clue is a double definition.
'piece of gym equipment' is the first definition.
(gymnastics equipment)
'one smile' is the second definition.
(to beam is to smile)
'making' acts as a link.
(Other definitions for beam that I've seen before include "Ship's width; ray" , "Smile; piece of timber" , "Shine" , "Piece of timber - shaft of light" , "Smile broadly or radiate" .)