One's voluntary contribution to the NHS may be of various types (5,5)
I believe the answer is:
blood donor
'be of various types' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'one's voluntary contribution to the nhs' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'odd' (I've seen this before) and 'odd' is present in the answer.
This explanation may well be incorrect...
'may' is the link.
Can you help me to learn more?
(Other definitions for blood donor that I've seen before include "whose contribution is vital" , "One giving to NHS" , "Enabler of transfusion" .)