Abandoned troopers pinching one's bottom (9)
I believe the answer is:
posterior
'bottom' is the definition.
(I know that bottom can be written as posterior)
'abandoned troopers pinching one's' is the wordplay.
'abandoned' indicates anagramming the letters (the original order is abandoned).
'pinching' indicates putting letters inside (taking inside).
'one' becomes 'i' (Roman numeral).
'troopers' anagrammed gives 'posteror'.
'posteror' placed around 'i' is 'POSTERIOR'.
(Other definitions for posterior that I've seen before include "Rear" , "Stern" , "Behind" , "later" , "Hindquarters" .)