Heartless trainer on field finds part of ear (7)
I believe the answer is:
cochlea
'part of ear' is the definition.
('cochlea' is part of 'ear')
'heartless trainer on field' is the wordplay.
'heartless' means to remove the middle letters.
'trainer' becomes 'coach' (coach is a kind of trainer).
'on' means one lot of letters go next to another.
'field' becomes 'lea' (I've seen this before).
'coach' with its middle removed is 'coch'.
'coch'+'lea'='COCHLEA'
'finds' is the link.
(Other definitions for cochlea that I've seen before include "A cloche (anag) - spiral cavity of the inner ear" , "Snail-shaped tube in the inner ear" , "Spiral tube in inner ear" , "located in the ear" , "there's a cavity, as we hear" .)