Allowing the removal of vital fluid? (7)
I believe the answer is:
letting
'allowing' is the definition.
('let' can be a synonym of 'allow')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for letting that I've seen before include "Allowing or leasing" , "Granting to a tenant" , "property for temporary occupants?" , "Giving permission to" , "Renting out; allowing" .)