A supportive smile? (4)
I believe the answer is:
beam
I believe this clue is a double definition.
'a supportive' is the first definition.
(I've seen this before)
'smile?' is the second definition.
(beam can mean to smile)
(Other definitions for beam that I've seen before include "Piece of timber - shaft of light" , "Sturdy timber" , "Light - heavy timber - wide smile" , "Broadcast" , "Perhaps a summer" .)