Doctors leave ring for insect (8)
I believe the answer is:
mosquito
'for insect' is the definition.
(mosquito is a kind of insect)
'doctors leave ring' is the wordplay.
'doctors' becomes 'MOS' (an MO is a Medical Officer).
'leave' becomes 'quit' (synonyms).
'ring' becomes 'O' (letter resembling a ring).
'mos'+'quit'+'o'='MOSQUITO'
(Other definitions for mosquito that I've seen before include "Annoying insect" , "Sucker" , "Malaria-carrying insect" , "Blood-sucker" , "Disease-bearing insect" .)