Climbs down support rails (10)

I believe the answer is:

backtracks

'climbs down' is the definition.
(climb down can mean to backtrack on something)

'support rails' is the wordplay.
'support' becomes 'back' (to back is to support or endorse).
'rails' becomes 'tracks' ('track' can be a synonym of 'rail').
'back'+'tracks'='BACKTRACKS'

I've seen this clue in The Guardian.

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