The most common and most likely answer to this crossword clue is the 5 letter word LEECH
I've seen this crossword clue in The Times
L
E
E
C
H
More clues leading to the result LEECH
- Shelter chief doctor of old
- Old doctor who’s after your blood?
- Hanger-on, a slippery creature, rejected companion
- Parasite finds shelter on top of church
- Sucker will disbelieve the basic truth, last of all
- Doctor's check after fish has been brought up
- End up defending conservative doctor once
- Cold in command to dog that’s brought back worm
- Sucker about to wear 8 shortly after getting up
- Fish about next to child’s old doctor
Buddy explains!

The straight part
We know that "Parasite" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case parasite = leech.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LEECH mean?
LEECH noun- An aquatic blood-sucking annelid of class Hirudinea, especially Hirudo medicinalis.
- A person who derives profit from others in a parasitic fashion.
- A glass tube designed for drawing blood from damaged tissue by means of a vacuum.
- To apply a leech medicinally, so that it sucks blood from the patient.
- To drain (resources) without giving back.
- A physician.
- (Heathenry) A healer.
- To treat, cure or heal.
- The vertical edge of a square sail.
- The aft edge of a triangular sail.