Synthetic material finally captured by photograph (7)
I believe the answer is:
plastic
'synthetic material' is the definition.
(I've seen this before)
'finally captured by photograph' is the wordplay.
'finally' becomes 'last' (synonyms).
'captured by' indicates putting letters inside.
'photograph' becomes 'pic' (short for picture).
'last' put inside 'pic' is 'PLASTIC'.
(Other definitions for plastic that I've seen before include "Credit cards are known as ....... money" , "Credit cards etc" , "source of credit" , "Credit card - pliable" , "Unattractively synthetic" .)