More complicated, fastening belt (8)

I believe the answer is:

knottier

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'more complicated' is the definition.
Both the answer and definition are comparative adjectives. Maybe they are linked in a way I don't understand?

'fastening belt' is the wordplay.
'fastening' becomes 'knot' (knot is a kind of fastening).
'belt' becomes 'tier' (I can't explain this - if you can you should give a lot more credence to this answer).
'knot'+'tier'='KNOTTIER'

Can you help me to learn more?

(Other definitions for knottier that I've seen before include "more puzzling" , "More tricky" .)

I've seen this clue in The Irish Times.

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