One challenging thing weighing on other ranks, initially (8)
I believe the answer is:
objector
'one challenging' is the definition.
'thing weighing on other ranks initially' is the wordplay.
'thing' becomes 'object' (I've seen this before**).
'weighing on' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'initially' indicates taking the first letters.
The initial letters of 'other ranks' is 'or'.
'object'+'or'='OBJECTOR'
(Other definitions for objector that I've seen before include "I oppose" , "One expressing opposition and disapproval" , "One protesting" , "one opposes" , "One stating opposition" .)