Large horn from Bath area (4)
I believe the answer is:
tuba
'large horn' is the definition.
(I've seen this in another clue)
'bath area' is the wordplay.
'bath' becomes 'tub' (synonyms).
'area' becomes 'a' (maths abbreviation).
'tub'+'a'='TUBA'
'from' acts as a link.
(Other definitions for tuba that I've seen before include "Large brass wind instrument" , "Brass instrument of low pitch" , "Brass "oompah" instrument" , "Brass band instrument" , "item band might play" .)