Left to the one that's getting on a bit (7)
I believe the answer is:
portion
'a bit' is the definition.
(thesaurus)
'left to the one that's getting on' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'to the one' becomes 'I' (Roman numeral).
'that's getting' means one lot of letters go next to another.
'port'+'i'+'on'='PORTION'
(Other definitions for portion that I've seen before include "Share, helping" , "Helping - fate" , "Piece of a whole" , "Serving" , "Part - share" .)