Ring dock about nothing (4)
I believe the answer is:
loop
'ring' is the definition.
(a loop is a ring of material)
'dock about nothing' is the wordplay.
'dock' becomes 'lop' (both can mean to cut).
'about' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'lop' going around 'o' is 'LOOP'.
(Other definitions for loop that I've seen before include "Oval or U-shaped coil or bend" , "Circular shape of pool" , "I cross myself" , "Endless film strip" , "Noose" .)