Nun with grin rambling on (7)

I believe the answer is:

running

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'on' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?

'nun with grin rambling' is the wordplay.
'with' says to put letters next to each other.
'rambling' is an anagram indicator.
'nun' after 'grin' is 'grinnun'.
'grinnun' anagrammed gives 'RUNNING'.

Can you help me to learn more?

(Other definitions for running that I've seen before include "cross-country event?" , "healthy activity?" , "in action" , "Managing; flowing" , "White City event" .)

I've seen this clue in The Sun.

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