Tennis match units (4)
I believe the answer is:
sets
'tennis match units' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sets that I've seen before include "Major divisions of tennis match" , "Solidifies" , "Scenery etc. of 11 across productions" , "Scenery etc. of dramatic productions" , "Groups of things of the same kind" .)