Doctor has a duty in time of want (7)
I believe the answer is:
drought
'time of want' is the definition.
(time when there is a scarcity or want of water)
'doctor has a duty' is the wordplay.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'has a duty' becomes 'ought' (I've seen this before).
'dr'+'ought'='DROUGHT'
'in' acts as a link.
(Other definitions for drought that I've seen before include "Aridity" , "Prolonged shortage of rain" , "Hot drug (anag)" , "dry condition" , "Period of abnormal dryness" .)