Casual shirt (3)
I believe the answer is:
tee
'casual shirt' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tee that I've seen before include "'Drive off it, half-laughing (3)'" , "Where a hole starts" , "Starting peg" , "Golf ball support" , "Golfing item" .)