End up defending conservative doctor once (5)
I believe the answer is:
leech
'doctor once' is the definition.
(leech is an archaic term for a doctor)
'end up defending conservative' is the wordplay.
'end' becomes 'heel' (heel can mean the end of something).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'defending' is an insertion indicator.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'heel' reversed gives 'leeh'.
'leeh' placed around 'c' is 'LEECH'.
(Other definitions for leech that I've seen before include "and one of his remedies" , "Parasite - hanger-on" , "Bloodsucker; early doctor" , "Blood-sucking worm or parasitical person" , "Freeloader" .)