Flexible mould used by cobblers in film (7)
I believe the answer is:
plastic
'flexible' is the definition.
(I've seen this in another clue)
'cobblers in film' is the wordplay.
'cobblers' becomes 'last' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'film' becomes 'pic' (short for picture).
'last' going into 'pic' is 'PLASTIC'.
'mould used by' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for plastic that I've seen before include "Synthetic substance" , "non-metallic material" , "Like some cards" , "Synthetic polymer" , "Synthetic material; credit cards (colloq.)" .)