Leave in the Aran knitting wool (6)
I believe the answer is:
angora
'wool' is the definition.
(I've seen this before)
'leave in the aran knitting' is the wordplay.
'leave' becomes 'go' (to go is to depart or leave).
'in the' is an insertion indicator.
'knitting' is an anagram indicator.
'aran' with letters rearranged gives 'anra'.
'go' put inside 'anra' is 'ANGORA'.
(Other definitions for angora that I've seen before include "Yarn or fabric made from goat hair" , "Long-haired cat, goat, or rabbit" , "Goat producing mohair" , "Wool from long-haired goats" , "Breed of goat, cat or rabbit" .)