Fine mat - a large saving (6)
I believe the answer is:
frugal
'saving' is the definition.
(saving money)
'fine mat a large' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'mat' becomes 'rug' (I've seen this before).
'large' becomes 'L' (eg in clothes sizes).
'f'+'rug'+'a'+'l'='FRUGAL'
(Other definitions for frugal that I've seen before include "Sparing, meagre" , "'Sparing, economical (6)'" , "Abstemious" , "Sparing with resources" , "not expensive" .)