Doctor's operation on pedestrian that's fallen (7)
I believe the answer is:
dropped
'that's fallen' is the definition.
(dropping is a kind of falling)
'doctor's operation on pedestrian' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'operation' becomes 'op'.
'on' says to put letters next to each other.
'pedestrian' becomes 'ped'.
'dr'+'op'+'ped'='DROPPED'
(Other definitions for dropped that I've seen before include "Left out of the team" , "Fell; abandoned" , "Relinquished" , "Let fall" .)