Popular retired doctor initially under such a cloud? (6)
I believe the answer is:
nimbus
'a cloud?' is the definition.
(nimbus is a kind of cloud)
'popular retired doctor initially under such' is the wordplay.
'popular' becomes 'in'.
'retired' shows that the letters should be reversed in order.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
'initially' says to take the initial letters.
The first letters of 'under such' is 'us'.
'in' back-to-front is 'ni'.
'ni'+'mb'+'us'='NIMBUS'
(Other definitions for nimbus that I've seen before include "showers under it" , "Ouse/Trent estuary" , "Is numb (anag)" , "Large grey rain cloud" , "Bums in (anag)" .)