Abandon doctor, sheltering horse he lost (7)
I believe the answer is:
forsake
'abandon' is the definition.
('forsake' can be a synonym of 'abandon')
'doctor sheltering horse he lost' is the wordplay.
'doctor' becomes 'fake' (**).
'sheltering' indicates putting letters inside.
'lost' indicates named letters should be taken away.
'horse' with 'he' taken away is 'ors'.
'fake' placed around 'ors' is 'FORSAKE'.
(Other definitions for forsake that I've seen before include "Leave in the lurch" , "'Abandon, desert (7)'" , "Give up" , "Abondon or renounce" .)