River boat ramming a masthead into bridge (6)
I believe the answer is:
sampan
'river boat' is the definition.
'sampan' can be an answer for 'boat' (sampan is a kind of boat). I am not certain of the 'river' bit.
'ramming a masthead into bridge' is the wordplay.
'ramming a masthead' becomes 'am' (I can't justify this - if you can you should believe this answer much more).
'into' is an insertion indicator.
'bridge' becomes 'span' (synonyms).
'am' inserted within 'span' is 'SAMPAN'.
Can you help me to learn more?
(Other definitions for sampan that I've seen before include "Small Chinese flat-bottomed boat" , "Small 10 boat" , "Small flat-bottomed boat in China" , "oriental vessel" , "Far Eastern boat" .)