Temper of doctor in the red (4)
I believe the answer is:
mood
'temper' is the definition.
('mood' can be a synonym of 'temper')
'doctor in the red' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'in the red' becomes 'od' (I've seen this before).
'mo'+'od'='MOOD'
'of' acts as a link.
(Other definitions for mood that I've seen before include "Low spirits" , "Frame of mind" , "Depression" , "Humour - bad temper" , "Doom (anag)" .)