Beware — fellow is a primitive (7)
I believe the answer is:
caveman
'a primitive' is the definition.
(caveman is a kind of primitive)
'beware fellow' is the wordplay.
'beware' becomes 'cave' (old-fashioned term meaning 'look out').
'fellow' becomes 'man' (I've seen this before).
'cave'+'man'='CAVEMAN'
'is' is the link.
(Other definitions for caveman that I've seen before include "Prehistoric person" , "Van came carrying old cliff dweller" , "Prehistoric male" , "old-timer?" , "Uncivilised chap" .)