Dandy fellow — or one whose contribution is vital (5,5)
I believe the answer is:
blood donor
'one whose contribution is vital' is the definition.
(I've seen this before)
'dandy fellow or' is the wordplay.
'dandy' becomes 'blood' (I can't explain this - if you can you should believe this answer much more).
'fellow' becomes 'don' (both can mean an academic).
'blood'+'don'+'or'='BLOOD-DONOR'
Can you help me to learn more?
(Other definitions for blood donor that I've seen before include "One giving to NHS" , "Enabler of transfusion" , "whose contribution is vital" .)