Doctor is required in absence of water (7)
I believe the answer is:
drought
'absence of water' is the definition.
(a drought is a shortage of water)
'doctor is required' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'is required' becomes 'ought' (if you ought to do something you're required to).
'dr'+'ought'='DROUGHT'
'in' acts as a link.
(Other definitions for drought that I've seen before include "Prolonged shortage of rainfall" , "Hot drug (anag)" , "Long dry spell" , "Lack of rainfall" , "Prolonged dry period" .)