Spirit — a small measure fellow’s consumed (6)
I believe the answer is:
daemon
'spirit' is the definition.
(I know that daemon is a type of spirit)
'a small measure fellow's consumed' is the wordplay.
'small measure' becomes 'em' (in typography, a space as wide as a letter 'm').
'fellow' becomes 'don' (both can mean an academic).
'consumed' means one lot of letters goes inside another (inserted letters get consumed).
'a'+'em'='aem'
'aem' put within 'don' is 'DAEMON'.
(Other definitions for daemon that I've seen before include "guardian spirit" , "Spirit in ancient Greek belief" , "Evil spirit" .)