My back, with constant tilt from one side to the other (4)
I believe the answer is:
rock
'one side to the other' is the definition.
I can't judge whether this definition defines the answer.
'my back with constant tilt' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'back' shows that the letters should be reversed in order.
'with constant tilt' becomes 'k' (I can't justify this - if you can you should believe this answer much more).
'cor' back-to-front is 'roc'.
'roc'+'k'='ROCK'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for rock that I've seen before include "Boulder - oscillate - sweetmeat" , "Reel from" , "Sway to and fro - mass of stone" , "Be unstable; lull (baby)" , "Big stone - music" .)