Hat to test new boy (7)
I believe the answer is:
stetson
'hat' is the definition.
(I know that Stetson is a type of hat)
'test new boy' is the wordplay.
'new' is an anagram indicator.
'boy' becomes 'son' (synonyms).
'test' is an anagram of 'stet'.
'stet'+'son'='STETSON'
'to' acts as a link.
(Other definitions for stetson that I've seen before include "Large hat" , "Ten-gallon hat" , "Hat, for country-and-western singer perhaps" , "A wide-brimmed cowboy hat" , "St Neots (anag.)" .)