Crossword Genius

A base insult badly open to being prolonged (11)

Ross

I believe the answer is:

sustainable

I'm a little stuck... Click here to teach me more about this clue!

'prolonged' is the definition.
Both the definition and answer are adjectives.
Perhaps they are linked in a way I don't understand?

'a base insult badly open to' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'open to' indicates putting letters inside (some letters open up to include others).
'base'+'insult'='baseinsult'
'baseinsult' with letters rearranged gives 'sustinable'.
'a' put into 'sustinable' is 'SUSTAINABLE'.

'being' acts as a link.

Can you help me to learn more?

(Other definitions for sustainable that I've seen before include "Able to continue over a period of time" , "It's a bus lane (anag.)" , "Maintaining a balance by not depleting natural resources" , "Able to be maintained" , "Able to be kept going" .)

I've seen this clue in The Times.
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