A fall involving work for the doctor (4)
I believe the answer is:
drop
'a fall' is the definition.
('drop' can be a synonym of 'fall')
'work for the doctor' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'for' means one lot of letters go next to another (I've seen this in other clues).
'the doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor'**).
'op' put after 'dr' is 'DROP'.
'involving' acts as a link.
(Other definitions for drop that I've seen before include "Desert" , "Dispense with - sink" , "Fall; tear" , "Collapse, globule" , "Tiny bit of liquid" .)