Fail to keep outside ring slack (5)
I believe the answer is:
loose
'slack' is the definition.
(I know that slack is a more specific form of loose)
'fail to keep outside ring' is the wordplay.
'fail to keep' becomes 'lose' (synonyms).
'outside' is an insertion indicator (some letters go outside others).
'ring' becomes 'O' (letter resembling a ring).
'lose' placed around 'o' is 'LOOSE'.
(Other definitions for loose that I've seen before include "Not firmly fixed" , "Not secured" , "Not tight" , "Not tied up or attached" , "Lax or at large" .)