Passed up holding girl's bottom (5)
I believe the answer is:
nadir
'bottom' is the definition.
(thesaurus)
'passed up holding girl's' is the wordplay.
'passed' becomes 'ran' (running is a kind of passing).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'holding' means one lot of letters goes inside another.
'girl' becomes 'di'.
'ran' written backwards gives 'nar'.
'nar' placed around 'di' is 'NADIR'.
(Other definitions for nadir that I've seen before include "The lowest point of anything" , "worst-case scenario?" , "Drain into the lowest point (5)" , "extremely low position" , "least successful point" .)