Position of top man back at last in grand vessel (8)
I believe the answer is:
kingship
'position of top man' is the definition.
(a king's position)
'back at last in grand vessel' is the wordplay.
'at last' suggests the final letters.
'grand' becomes 'G' (G is short for ground as in a thousand).
'vessel' becomes 'ship' (ship is a kind of vessel).
The last letter of 'back' is 'k'.
'k'+'in'+'g'+'ship'='KINGSHIP'
(Other definitions for kingship that I've seen before include "Sovereignty" , "Monarch's status" , "palace office" , "Royalty" , "Sovereign's office" .)