Top three sit out (3,5)
I believe the answer is:
tee shirt
'top' is the definition.
(I know that tee-shirt is a type of shirt)
'three sit out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'three'+'sit'='threesit'
'threesit' anagrammed gives 'TEE-SHIRT'.
(Other definitions for tee shirt that I've seen before include "casual wear" , "Simple garment" , "Casual garment" , "top?" .)