Professor alternatively joins sponsor (5)
I believe the answer is:
donor
'sponsor' is the definition.
(I've seen this before)
'professor alternatively joins' is the wordplay.
'professor' becomes 'don' (term for a university professor).
'alternatively' becomes 'or'.
'joins' means one lot of letters go next to another.
'don'+'or'='DONOR'
(Other definitions for donor that I've seen before include "Someone providing" , "Organ giver?" , "blessed person, relatively" , "One giving something" , "contributor" .)