Disease vector medical officers leave out at first (8)
I believe the answer is:
mosquito
'disease vector' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'medical officers leave out at first' is the wordplay.
I cannot really see how this works, but
'leave' could be 'quit' (quitting is a kind of leaving) and 'quit' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for mosquito that I've seen before include "Disease-bearing insect" , "Disease-spreading insect" , "Blood-sucking insect" , "Malaria carrier" , "Plane used in WWII" .)