Secure, since containing synthetic polymer (5)

I believe the answer is:

resin

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'synthetic polymer' is the definition.
'resin' can be an answer for 'polymer' (I've seen this before). I am not certain of the 'synthetic' bit.

'secure since containing' is the wordplay.
'containing' indicates a hidden word.
'RESIN' is hidden within 'secure since'.

Can you help me to learn more?

(Other definitions for resin that I've seen before include "Sticky material" , "Elemi, for example" , "Lac" , "Origin of amber" , "Material exuded from tree" .)

I've seen this clue in The Daily Express.

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