Mostly stolen, deficit's original solution for the most senior (6)
I believe the answer is:
oldest
'most senior' is the definition.
('old' can be similar in meaning to 'senior')
'mostly stolen deficit's original solution for the' is the wordplay.
'mostly stolen deficit's' becomes 'olde' (I can't explain this - if you can you should believe this answer much more).
'original' suggests taking the first letters.
'for' is a charade indicator (letters next to each other).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
The initial 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" .)