This person's left with soldier leading (9)
I believe the answer is:
important
'leading' is the definition.
(I've seen this before)
'this person's left with soldier' is the wordplay.
'this person's' becomes 'im' (this person means the crossword setter themselves).
'left' becomes 'port' (left side of a ship - opposite of starboard**).
'with' says to put letters next to each other.
'soldier' becomes 'ant' (I've seen this before).
'im'+'port'+'ant'='IMPORTANT'
(Other definitions for important that I've seen before include "Of significance or value" , "Main" , "Noteworthy, significant" , "A priority" , "Historic" .)