Get away with a summit, regularly cancelled, being put back (5)
I believe the answer is:
imshi
'get away' is the definition.
(Australian slang term meaning 'go away')
'with a summit regularly cancelled being put back' is the wordplay.
'regularly cancelled' indicates alternate letters (letters removed at regular intervals).
'being put back' is a reversal indicator.
'with'+'a'+'summit'='withasummit'
The alternating letters of 'withasummit' are 'ihsmi'.
'ihsmi' written backwards gives 'IMSHI'.
(Other definitions for imshi that I've seen before include "leave" , "Go away" , "Depart for Arabia" .)