A lot of tennis outfits (4)
I believe the answer is:
sets
I believe this is a double definition.
'a lot of tennis' is the first definition.
(more than one tennis set)
'outfits' is the second definition.
(set can mean a set of clothing)
(Other definitions for sets that I've seen before include "Plants" , "Coagulates" , "Scenery etc. of 11 across productions" , "Classes" , "Solidifies" .)