At intervals, turn wheel ... lap of Mull futile (9)
I believe the answer is:
unhelpful
'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" .)