Sort of shirt, one of a set of eighteen? (3)
I believe the answer is:
tee
I believe this is a double definition.
'sort of shirt' is the first definition.
(T-shirt)
'one of a set of eighteen?' is the second definition.
(18 tees for 18 holes on a golf course)
(Other definitions for tee that I've seen before include "Golfing support" , "Where one drives from" , "Stand" , "Starting place on the golf course" , "Golfing equipment" .)