Synthetic material shown by model in illustration (7)
I believe the answer is:
plastic
'synthetic material' is the definition.
(I've seen this before)
'model in illustration' is the wordplay.
'model' becomes 'last' (model used by a shoemaker).
'in' indicates putting letters inside.
'illustration' becomes 'pic' (short for picture).
'last' put within 'pic' is 'PLASTIC'.
'shown by' is the link.
(Other definitions for plastic that I've seen before include "Capable of being moulded" , "Mouldable material" , "(Originally malleable) material" , "Credit card - pliable" , "Flexible - synthetic material" .)