Extent of responsibility left one fool struggling (9)
I believe the answer is:
portfolio
'extent of responsibility' is the definition.
'left one fool struggling' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard**).
'one' becomes 'i' (Roman numeral).
'struggling' indicates anagramming the letters.
'i'+'fool'='ifool'
'ifool' with letters rearranged gives 'folio'.
'port'+'folio'='PORTFOLIO'
(Other definitions for portfolio that I've seen before include "Flat case for paper sheets" , "A set of investment" , "Minister's responsibility" , "artist's sample work" , "Collection of papers" .)