Modifiable little picture, finally included (7)
I believe the answer is:
plastic
'modifiable' is the definition.
(thesaurus)
'little picture finally included' is the wordplay.
'little picture' becomes 'pic'.
'finally' becomes 'last' (synonyms).
'included' is an insertion indicator.
'pic' placed around 'last' is 'PLASTIC'.
(Other definitions for plastic that I've seen before include "Ubiquitous synthetic material" , "Artificial/synthetic" , "Unattractively synthetic" , "Like some cards" , "Credit card - pliable" .)