Crossword Genius

At intervals, turn wheel ... lap of Mull futile (9)

Ross

I believe the answer is:

unhelpful

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'futile' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?

'at intervals turn wheel lap of mull' is the wordplay.
'at intervals' indicates alternate letters (letters are taken at regular intervals).
'turn'+'wheel'+'lap'+'of'+'mull'='turnwheellapofmull'
The alternate letters of 'turnwheellapofmull' are 'UNHELPFUL'.

Can you help me to learn more?

(Other definitions for unhelpful that I've seen before include "disobliging" , "Not of assistance, probably the opposite" , "One doesn't expect a daily to be so" , "Not co-operative or of assistance" , "Not constructive" .)

I've seen this clue in The Independent.
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