One goes round castle, treading nothing into carpet (4)
I believe the answer is:
moat
'one goes round castle' is the definition.
(a castle might be surrounded by a moat)
'nothing into carpet' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'into' indicates putting letters inside.
'carpet' becomes 'mat' (a mat is a floor covering).
'o' going within 'mat' is 'MOAT'.
'treading' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for moat that I've seen before include "Trench round a castle" , "Castle's ditch" , "Castle surround" , "Trench around fortifications" , "quantity of water" .)