Illustration carrying little weight with a girl (7)
I believe the answer is:
diagram
'illustration' is the definition.
(I know that illustration can be written as diagram)
'carrying little weight with a girl' is the wordplay.
'carrying' becomes 'a' (I can't justify this - if you can you should believe this answer much more).
'little weight' becomes 'gram'.
'with' says to put letters next to each other.
'a girl' becomes 'Di'.
'a'+'gram'='agram'
'agram' put after 'di' is 'DIAGRAM'.
Can you help me to learn more?
(Other definitions for diagram that I've seen before include "Explanatory illustration" , "Explanatory figure" , "Simplified drawing" , "scheme" , "A grid, Ma? (Anag)" .)