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