Vital I'm left with worker (9)
I believe the answer is:
important
'vital' is the definition.
(thesaurus)
'i'm left with worker' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'with' is a charade indicator (letters next to each other).
'worker' becomes 'ant' (worker is a type of ant).
'im'+'port'+'ant'='IMPORTANT'
(Other definitions for important that I've seen before include "influential" , "Main" , "Leading" , "A priority" , "It's vital" .)