It calls for water, as a doctor should (7)
I believe the answer is:
drought
'it calls for water' is the definition.
The definition suggests a singular noun which matches the answer.
'a doctor should' is the wordplay.
'a doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'should' becomes 'ought' ('ought' can be a synonym of 'should').
'dr'+'ought'='DROUGHT'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for drought that I've seen before include "Severe water shortage" , "dry condition" , "Aridity" , "Long dry spell" , "Prolonged shortage of rainfall" .)