Mostly stolen, deficit's original solution for the most senior (6)
I believe the answer is:
oldest
'most senior' is the definition.
('old' is associated in meaning with 'senior')
'mostly stolen deficit's original solution for the' is the wordplay.
'mostly stolen deficit' becomes 'olde' (I can't justify this - if you can you should believe this answer much more).
'original' indicates taking the first letters.
'for' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
The first letter of 'solution' is 's'.
'olde'+'s'+'t'='OLDEST'
Can you help me to learn more?
(Other definitions for oldest that I've seen before include "Least young" , "Least recent" , "Let's do! (anag.)" , "Most ancient (O, not E)" , "Eldest" .)