Altered rocking lever (7)
I believe the answer is:
treadle
'lever' is the definition.
(treadle is a kind of lever)
'altered rocking' is the wordplay.
'rocking' indicates an anagram.
'altered' anagrammed gives 'TREADLE'.
(Other definitions for treadle that I've seen before include "driving mechanism" , "Foot-operated lever" , "Foot lever working machine" , "Pedal operating machine" , "Related (anag.)" .)