Large snake’s scaling device (6)
I believe the answer is:
ladder
'scaling device' is the definition.
(I've seen this before)
'large snake's' is the wordplay.
'large' becomes 'l'.
'snake' becomes 'adder' (adder is a kind of snake).
'l'+'adder'='LADDER'
(Other definitions for ladder that I've seen before include "Scaling device" , "sort of tournament" , "Climbing aid" , "Climbing structure" , "A fire-fighter may use one" .)