A stiffener? You can get one at the Boar’s Head (4)
I believe the answer is:
bone
'a stiffener? you' is the definition.
The definition suggests a singular noun which matches the answer.
'one at the boar's head' is the wordplay.
'at' means one lot of letters go next to another (I've seen this in other clues).
'head' indicates taking the first letters.
The initial letter of 'boars' is 'b'.
'one' put after 'b' is 'BONE'.
'can get' acts as a link.
Can you help me to learn more?
(Other definitions for bone that I've seen before include "Skeletal segment" , "Skeleton material" , "How dry" , "Ossified material" , "Scapula, radius or tibia?" .)