Stay tough as opposition become weak... (2,4)

I believe the answer is:

go soft

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'stay tough as opposition' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'become weak' is the wordplay.
'become' becomes 'go' (eg to go rotten is to become rotten).
'weak' becomes 'soft' (similar in meaning).
'go'+'soft'='GO SOFT'

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I've seen this clue in The Irish Times.

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