Bridge is blocked by American boat (6)
I believe the answer is:
sampan
'boat' is the definition.
(I know that sampan is a type of sailing vessel)
'bridge is blocked by american' is the wordplay.
'bridge' becomes 'span' (synonyms).
'is blocked by' is an insertion indicator (I've seen this in other clues).
'american' becomes 'Am' (abbreviation in some contexts).
'span' enclosing 'am' is 'SAMPAN'.
(Other definitions for sampan that I've seen before include "oriental vessel" , "Small Asian rowing boat" , "Small 10 boat" , "Small oriental rowing boat" , "Flat-bottomed Chinese skiff" .)