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Thwart
fickle
rule
in
gear
shift?
(10)
Thwart fickle rule in gear shift? (10)
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The definition is at the end of the clue.
How the clue works
Have a look at gear shift? at the end there — that's your definition, a derailleur being the bit on a bike that shifts the gears. To build it, you want a word for 'thwart', which is DERAIL, then tack on 'rule' scrambled up (that's what 'fickle' is telling you to do) to give LEUR.