Scottish topper has Mat's back (3)
I believe the answer is:
tam
'scottish topper' is the definition.
(I've seen this in another clue)
'mat's back' is the wordplay.
'back' says the letters should be written in reverse.
'mat' backwards is 'TAM'.
'has' is the link.
(Other definitions for tam that I've seen before include "Headgear" , "Index rating for TV watching" , "Television ratings system" , "Turn over the mat for a cap" , "...-o'-shanter, Scottish cap" .)