Not above breaking an elbow (5)
I believe the answer is:
below
'not above' is the definition.
('below' is the opposite to 'above')
'breaking an elbow' is the wordplay.
'breaking' indicates an anagram.
'elbow' with letters rearranged gives 'BELOW'.
(Other definitions for below that I've seen before include "Later in text" , "Feel miserable" , "on earth" , "Inferior" , "Underneath" .)