Continue to think, going back through again (6)
I believe the answer is:
resume
'continue' is the definition.
(resuming is a kind of continuing)
'think going back through again' is the wordplay.
'think' becomes 'muse' (musing is a kind of thinking**).
'going back' says the letters should be written backwards.
'through' is an insertion indicator (I've seen this in other clues).
'again' becomes 're'.
'muse' written backwards gives 'esum'.
'esum' put within 're' is 'RESUME'.
'to' acts as a link.
(Other definitions for resume that I've seen before include "Start again where one left off" , "Begin activity again" , "Carry on (with something adjourned)" , "Start or take up again" , "Carry on where one left off" .)