Fellow doctor crossing old hillside hollow (5)
I believe the answer is:
coomb
'hillside hollow' is the definition.
(a coomb is a valley on a hillside)
'fellow doctor crossing old' is the wordplay.
I cannot really understand how this works, but
'fellow' could be 'co' (eg a co-worker is a fellow worker) and 'co' is located in the answer.
'doctor' could be 'mb' (Bachelor of Medicine, Medicinae Baccalaureus) and 'mb' is located in the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for coomb that I've seen before include "Hollow on hill" , "Short valley or hollow" , "Hillside valley or hollow" .)