Disappointed expression of Finn, perhaps, aboard vessel (6)
I believe the answer is:
shucks
'disappointed expression' is the definition.
(shucks is an expression of disappointment)
'finn perhaps aboard vessel' is the wordplay.
'finn perhaps' becomes 'Huck' (Huckleberry Finn is a fictional character).
'aboard' is an insertion indicator.
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'huck' going inside 'ss' is 'SHUCKS'.
'of' is the link.
(Other definitions for shucks that I've seen before include "Oyster shells" , "Exclamation of disappointment - husks or pods" , "cry of disappointment" , "Expression of disappointment" , "Exclamation of disappointment or surprise" .)