Go down round circle and let go (5)
I believe the answer is:
loose
'let go' is the definition.
(loosing is a kind of letting go)
'go down round circle' is the wordplay.
'go down' becomes 'lose' (I've seen this before).
'round' indicates putting letters inside.
'circle' becomes 'O' (O is a circle).
'lose' enclosing 'o' is 'LOOSE'.
'and' is the link.
(Other definitions for loose that I've seen before include "Not strict" , "Not restrained or attached" , "poor fit?" , "Unsecured; dissolute" , "Unattached" .)