Model fellow in back with family (8)
I believe the answer is:
mannikin
'model' is the definition.
'fellow in back with family' is the wordplay.
'fellow' becomes 'man' (fellow can mean a man).
'back' says the letters should be written backwards.
'with' means one lot of letters go next to another.
'family' becomes 'kin' (synonyms).
'in' backwards is 'ni'.
'man'+'ni'+'kin'='MANNIKIN'
(Other definitions for mannikin that I've seen before include "Clothes dummy" , "minor fellow" , "Little chap" , "Anatomical model - dwarf - child" .)