About to tour St Petersburg once, start off in reverse (10)
I believe the answer is:
retrograde
'reverse' is the definition.
(I've seen this before)
'about to tour st petersburg once start off' is the wordplay.
'about' becomes 're' (common abbreviation for 'with reference to').
'to tour' is an insertion indicator (to tour can mean to go around).
'st petersburg once' becomes 'Petrograd' (former name of St Petersburg).
'start off' means to remove the first letter (I've seen 'starting off' mean this (the starting letter is removed)).
'petrograd' with its initial letter taken off is 'etrograd'.
're' enclosing 'etrograd' is 'RETROGRADE'.
'in' acts as a link.
(Other definitions for retrograde that I've seen before include "Reactionary" , "Moving backwards" , "degenerate" , "Declining" , "Reverse" .)