Blood-suckers left runny cheese (7)
'blood-suckers' is the definition.
(I've seen this before)
'left runny cheese' is the wordplay.
I cannot quite see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found in the answer.
an anagram of 'cheese' is 'eeches' which is located in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for leeches that I've seen before include "drains resources", "Bloodsuckers", "Physicians once", "Bloodsucking parasites", "Blood-sucking worms once used by doctors".)