Cor! Upset with monarch moving back and forth (7)
I believe the answer is:
rocking
'moving back and forth' is the definition.
(rocking is a kind of moving back and forth)
'cor upset with monarch' is the wordplay.
'upset' is a reversal indicator.
'with' means one lot of letters go next to another.
'monarch' becomes 'king' (king is a kind of monarch).
'cor' written backwards gives 'roc'.
'roc'+'king'='ROCKING'
(Other definitions for rocking that I've seen before include "Tilting from side to side" , "Shocking" , "Moving from side to side" , "dancing!" , "... chair, one that moves back and forth" .)