Fur for sale outside the Boar's Head (5)
I believe the answer is:
sable
'fur' is the definition.
(sable is a kind of fur)
'sale outside the boar's head' is the wordplay.
'outside' indicates putting letters inside.
'head' suggests taking the first letters.
The initial letter of 'boars' is 'b'.
'sale' placed around 'b' is 'SABLE'.
'for' is the link.
(Other definitions for sable that I've seen before include "Adventurous courage" , "North Asian marten" , "Brush" , "Fur from member of weasel family" , "Bales of valuable Russian fur" .)