Directions get tough, but not too bad (9)

I believe the answer is:

endurable

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'not too bad' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?

'directions get tough' is the wordplay.
'directions' becomes 'en' (I've seen this in other clues).
'get' says to put letters next to each other.
'tough' becomes 'durable' (I've seen this in another clue).
'en'+'durable'='ENDURABLE'

'but' acts as a link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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