One terribly boring fat man? (7)
I believe the answer is:
'man?' is the definition.
'one terribly boring fat' is the wordplay.
'terribly' indicates anagramming the letters.
'boring' is an insertion indicator (some letters must 'bore' a hole into other letters).
'fat' becomes 'lard' (I've seen this before).
'one' anagrammed gives 'eon'.
'eon' put inside 'lard' is 'LEONARD'.
(Other definitions for leonard that I've seen before include "-- Cohen, singer" , "-- Bernstein, US conductor" , "Fellow" , "man" , "Elmore ___ , US thriller writer, d. 2013" .)