Left one building up steps (6)
I believe the answer is:
ladder
'steps' is the definition.
(ladder is a kind of steps)
'left one building up' is the wordplay.
'left' becomes 'l' (common abbreviation).
'one building up' becomes 'adder' (I can't explain this - if you can you should believe this answer much more).
'l'+'adder'='LADDER'
Can you help me to learn more?
(Other definitions for ladder that I've seen before include "Climbing aid" , "Household climbing device or hole in stocking" , "Vertical run in tights" , "Domestic device for climbing" , "Household climbing device or tear in stocking" .)