Top left aboard boat (8)
I believe the answer is:
pinnacle
'top' is the definition.
(I know that pinnacle is a type of top)
'left aboard boat' is the wordplay.
'left' becomes 'l' (common abbreviation).
'aboard' indicates putting letters inside.
'boat' becomes 'pinnace' (historical type of small boat).
'l' placed into 'pinnace' is 'PINNACLE'.
(Other definitions for pinnacle that I've seen before include "High point of rock" , "High pointed piece of rock" , "The highest level" , "Acme" , "tip-top" .)