Like rosin, it can make reins flexible (5)

I believe the answer is:

resin

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'like rosin' is the definition.
'resin' can be an answer for 'rosin' (I have seen 'eg Rosin ' mean 'resin' so perhaps 'rosin' could also mean 'resin'). I'm not sure about the 'like' bit.

'it can make reins flexible' is the wordplay.
'reins' anagrams to 'RESIN'.
Though, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for resin that I've seen before include "Fiddler's need" , "component of varnish" , "Contributor to plastics industry" , "Glue" , "stuff that's sticky" .)

I've seen this clue in the Evening Standard.

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