Hardens (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
sets
'hardens' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sets that I've seen before include "Solidifies" , "Scenery etc. of 11 across productions" , "Major divisions of tennis match" , "Scenery etc. of dramatic productions" , "Hardens" .)