Tennis units (4)
I believe the answer is:
sets
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tennis units</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for sets that I've seen before include "Hardens" , "Groups of things of the same kind" , "Scenery etc. of dramatic productions" , "Major divisions of tennis match" , "Classes" .)