Am, in the snap, going for a swim from the vessel (6)
I believe the answer is:
sampan
'the vessel' is the definition.
(sampan is a kind of vessel)
'am in the snap going for a swim' is the wordplay.
I cannot quite understand how this works, but
'am' is within the answer.
an anagram of 'snap' is 'span' which is located in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'from' acts as a link.
Can you help me to learn more?
(Other definitions for sampan that I've seen before include "Small Asian rowing boat" , "Far Eastern boat" , "Small flat-bottomed boat in China" , "Asian skiff" , "Small 10 boat" .)