Not taut; loose (5)
I believe the answer is:
slack
I believe this is a double definition.
'not taut' is the first definition.
(I've seen this before)
'loose' is the second definition.
(I know that loose can be written as slack)
(Other definitions for slack that I've seen before include "Idle and disinclined to work" , "Limp - negligent" , "Not taut" , "skive" , "Flaccid" .)