Stay in, worried for soundness of mind (6)
I believe the answer is:
sanity
'soundness of mind' is the definition.
(I've seen this before)
'stay in worried' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'stay'+'in'='stayin'
'stayin' is an anagram of 'SANITY'.
'for' is the link.
(Other definitions for sanity that I've seen before include "Mental health" , "presence of mind?" , "State of having a healthy mind" , "Soundness of mind" , "Healthy mental state" .)