No snake on board but a large viper! (6)
I believe the answer is:
ladder
'no snake on board' is the definition.
I can't judge whether this definition defines the answer.
'a large viper' is the wordplay.
'a large' becomes 'L' (eg in clothes sizes).
'viper' becomes 'adder' (adder is a kind of viper).
'l'+'adder'='LADDER'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for ladder that I've seen before include "Run in stockings" , "snag in hose" , "Set of steps" , "Domestic device for climbing" , "Framework for scaling, usually portable" .)