Desire to get firm with doctor (5)
I believe the answer is:
covet
'desire' is the definition.
(I've seen this before)
'firm with doctor' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with' means one lot of letters go next to another.
'doctor' becomes 'vet' (vet is a kind of doctor).
'co'+'vet'='COVET'
'to get' is the link.
(Other definitions for covet that I've seen before include "Hanker after" , "Yearn to possess (an item owned by another)" , "Crave something belonging to someone else" , "Want something belonging to another" , "'Thou shalt not . . . . . thy neighbour's goods'" .)