Get big twig (5,2)

I believe the answer is:

catch on

I believe this clue is a double definition.

'get big' is the first definition.
(as in a trend catching on or getting big)

'twig' is the second definition.
(synonyms)

(Other definitions for catch on that I've seen before include "Informally, become popular or come to understand" , "twig" , "Get it" , "Understand - become fashionable" .)

I've seen this clue in The Times.

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