Straighten out joint with doctor (4)
I believe the answer is:
comb
'straighten out' is the definition.
(combing is a kind of straightening out)
'joint with doctor' is the wordplay.
'joint' becomes 'co' (eg a co-leader is a joint leader).
'with' is a charade indicator (letters next to each other).
'doctor' becomes 'MB' (Bachelor of Medicine).
'co'+'mb'='COMB'
(Other definitions for comb that I've seen before include "Toothed grooming implement" , "Crest on the head of some birds" , "Go over" , "Rummage through" , "Search methodically" .)