Open requires balls on tee (5)
I believe the answer is:
overt
'open' is the definition.
(synonyms)
'balls on tee' is the wordplay.
'balls' becomes 'over' (I've seen this in other clues).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'tee' becomes 't'.
'over'+'t'='OVERT'
'requires' acts as a link.
(Other definitions for overt that I've seen before include "Not locked away" , "Obvious, patent" , "Open, up front" , "not disguised" , "Open and unconcealed" .)