Boat covering length to attain point (8)
I believe the answer is:
pinnacle
'point' is the definition.
(pinnacle is a kind of point)
'boat covering length' is the wordplay.
'boat' becomes 'pinnace' (historical type of small boat).
'covering' indicates putting letters inside.
'length' becomes 'l'.
'pinnace' enclosing 'l' is 'PINNACLE'.
'to attain' is the link.
(Other definitions for pinnacle that I've seen before include "The highest point, say of fame" , "High pointed piece of rock" , "The highest point, summit" , "Apex" , "Height of success" .)