Fashionable artist frames them regularly, that's for sure (13)

I believe the answer is:

incontestable

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

'fashionable' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

'artist frames them regularly that's for sure' is the wordplay.
'artist frames' becomes 'incon' (I can't explain this - if you can you should give a lot more credence to this answer).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'that's for' says to put letters next to each other.
'sure' becomes 'stable'.
The alternating letters of 'them' are 'te'.
'incon'+'te'+'stable'='INCONTESTABLE'

Can you help me to learn more?

(Other definitions for incontestable that I've seen before include "Beyond dispute" , "sure" , "Without doubt" .)

I've seen this clue in The Times.

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