Perhaps sizeable snake transported at first by vessel launched from merchant ship? (8)
I believe the answer is:
longboat
'vessel launched from merchant ship?' is the definition.
I can't tell whether this defines the answer.
'perhaps sizeable snake transported at first' is the wordplay.
'perhaps sizeable' becomes 'long' (I can't explain this - if you can you should give a lot more credence to this answer).
'snake' becomes 'boa' (boa is a kind of snake).
'at first' says to take the initial letters.
The initial letter of 'transported' is 't'.
'long'+'boa'+'t'='LONGBOAT'
'by' acts as a link.
Can you help me to learn more?
(Other definitions for longboat that I've seen before include "Ship's largest vessel" , "one attacking from the north" , "Vessel used by the Vikings" , "one getting a row?" , "Large vessel launched from a sailing ship" .)