Modify rump mostly used in place of another (9)
I believe the answer is:
alternate
'in place of another' is the definition.
I can't judge whether this defines the answer.
'modify rump mostly' is the wordplay.
'modify' becomes 'alter' (I've seen this before).
'rump mostly' becomes 'nate' (I can't justify this - if you can you should believe this answer much more).
'alter'+'nate'='ALTERNATE'
'used' acts as a link.
Can you help me to learn more?
(Other definitions for alternate that I've seen before include "go back and forth" , "Flip" , "Occurring in turn repeatedly" , "deputy" , "Every second one of a series" .)