Changed terrible treadle (7)
I believe the answer is:
altered
'changed' is the definition.
('alter' can be a synonym of 'change')
'terrible treadle' is the wordplay.
'terrible' indicates an anagram.
'treadle' is an anagram of 'ALTERED'.
(Other definitions for altered that I've seen before include "Treadle (anag)" , "Changed, modified" , "Changed, adapted" , "Change" , "Converted" .)