Left snake under steps (6)
I believe the answer is:
ladder
'steps' is the definition.
(ladder is a kind of steps)
'left snake under' is the wordplay.
'left' becomes 'l' (common abbreviation).
'snake' becomes 'adder' (adder is a kind of snake).
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'l'+'adder'='LADDER'
(Other definitions for ladder that I've seen before include "See 12" , "Companionway" , "ranking table" , "Row of unravelled stitches" , "sometimes with companion" .)