Tailors are, not entirely surprisingly, relating to clothes (9)
I believe the answer is:
sartorial
'relating to clothes' is the definition.
'tailors are not entirely surprisingly' is the wordplay.
'not entirely' means to remove the last letter.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
'tailors'+'are'='tailorsare'
'tailorsare' with its last letter taken off is 'tailorsar'.
'tailorsar' with letters rearranged gives 'SARTORIAL'.
(Other definitions for sartorial that I've seen before include "Styled" , "Pertaining to dress" , "Relating to tailoring" , "for a fitting person" , "About tailoring men's clothes, may possess elegance" .)