Fellow student unfortunately came last, son claimed (9)
I believe the answer is:
classmate
'fellow student' is the definition.
(I've seen this in another clue)
'unfortunately came last son claimed' is the wordplay.
'unfortunately' is an anagram indicator (a bad or unfortunate spelling).
'son' becomes 's' (genealogical abbreviation for son).
'claimed' means one lot of letters goes inside another (inserted letters are claimed by another word).
'came'+'last'='camelast'
'camelast' is an anagram of 'clasmate'.
'clasmate' enclosing 's' is 'CLASSMATE'.
(Other definitions for classmate that I've seen before include "Fellow pupil" , "Fellow student" , "School associate" , "one of the others being in form" , "Schoolfellow" .)