Small automobile with loud muffler (5)
I believe the answer is:
scarf
'muffler' is the definition.
(I've seen this before)
'small automobile with loud' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'automobile' becomes 'car' (synonyms).
'with' says to put letters next to each other.
'loud' becomes 'f' (short for forte - 'loud' in music).
's'+'car'+'f'='SCARF'
(Other definitions for scarf that I've seen before include "One round neck; a wood joint" , "Quickly finish off" , "Neck covering" , "Neck/head garment" , "Winter wear" .)