Mislay a ring inside when slack (5)
I believe the answer is:
loose
'slack' is the definition.
(I know that slack can be written as loose)
'mislay a ring inside' is the wordplay.
'mislay' becomes 'lose' (synonyms).
'a ring' becomes 'o' (letter resembling a ring).
'inside' is an insertion indicator.
'lose' going around 'o' is 'LOOSE'.
'when' is the link.
(Other definitions for loose that I've seen before include "Not tight" , "wobbly" , "Not crammed" , "Not firmly fixed" , "Not restrained or attached" .)