Southern passage held the fort, making up deficiency (9)
I believe the answer is:
shortfall
'up deficiency' is the definition.
'shortfall' can be an answer for 'deficiency' (shortfall is a kind of deficiency). I am not sure about the 'up' bit.
'southern passage held the fort making' is the wordplay.
'southern' becomes 's' (abbreviation).
'passage' becomes 'hall' (hall is a kind of passage).
'held' indicates putting letters inside.
'making' indicates anagramming the letters.
'fort' with letters rearranged gives 'ortf'.
'hall' placed around 'ortf' is 'hortfall'.
's'+'hortfall'='SHORTFALL'
Can you help me to learn more?
(Other definitions for shortfall that I've seen before include "Loss" , "what we're lacking" , "Not quite what's expected" , "Not enough money" , "Deficiency" .)