Test out boy for a hat (7)
I believe the answer is:
stetson
'for a hat' is the definition.
(Stetson is an example)
'test out boy' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'boy' becomes 'son' (synonyms).
'test' is an anagram of 'stet'.
'stet'+'son'='STETSON'
(Other definitions for stetson that I've seen before include "Man's hat with wide brim and high crown" , "St Neots (anag.)" , "Tip" , "A wide-brimmed cowboy hat" , "Capital investment" .)