Revolutionary fled pinching princess' bottom (5)
I believe the answer is:
nadir
'bottom' is the definition.
(I've seen this before)
'revolutionary fled pinching princess'' is the wordplay.
'revolutionary' says the letters should be written in reverse.
'fled' becomes 'ran' (I've seen this before).
'pinching' means one lot of letters goes inside another (taking inside).
'princess'' becomes 'di' (Princess Diana).
'ran' back-to-front is 'nar'.
'nar' placed around 'di' is 'NADIR'.
(Other definitions for nadir that I've seen before include "Low spot" , "All-time low" , "worst-case scenario?" , "Rock bottom" , "least successful point" .)