Gels (4)
'gels' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sets that I've seen before include "Places" , "Hardens" , "Scenery etc. of dramatic productions" , "Units of play in tennis" , "Scenery etc. of 11 across productions" .)
(Another definition for firms that I've seen is " Companies".)