Ulster fellow on floor is slicker (7)

I believe the answer is:

niftier

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'slicker' is the definition.
Both the answer and definition are comparative adjectives. Perhaps you can see a link between them that I can't see?

'ulster fellow on floor' is the wordplay.
'ulster' becomes 'NI' (province of Ireland containing Northern Ireland).
'fellow' becomes 'F' (abbreviation - of a society etc.).
'on floor' becomes 'tier' (I am not sure about this - if you are sure you should believe this answer much more).
'ni'+'f'+'tier'='NIFTIER'

'is' is the link.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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