Reject skipper essentially going aboard small vessel (5)
I believe the answer is:
spurn
'reject' is the definition.
(I know that reject can be written as spurn)
'skipper essentially going aboard small vessel' is the wordplay.
'essentially' means to look at the middle letters.
'going aboard' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vessel' becomes 'urn' (urn is a kind of vessel).
The centre of 'skipper' is 'p'.
's'+'urn'='surn'
'p' going into 'surn' is 'SPURN'.
(Other definitions for spurn that I've seen before include "turn away" , "Repudiate" , "Reject scornfully, rebuff" , "Slight" , "Despise" .)