Hearing aid company chain, Field (7)
I believe the answer is:
cochlea
'hearing aid' is the definition.
(part of the ear)
'company chain field' is the wordplay.
'company' becomes 'co' (abbreviation).
'chain' becomes 'ch' (abbreviation for the unit of length).
'field' becomes 'lea' (lea can mean a meadow or field of grass).
'co'+'ch'+'lea'='COCHLEA'
(Other definitions for cochlea that I've seen before include "Part of the inner ear" , "Spiral tube in inner ear" , "located in the ear" , "Coiled tube in the inner ear" , "A cloche (anag) - spiral cavity of the inner ear" .)