A sickly brunette's out for reassurance (8,7)
I believe the answer is:
security blanket
'reassurance' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'a sickly brunette's out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'a'+'sickly'+'brunette'='asicklybrunette'
'asicklybrunette' is an anagram of 'SECURITY BLANKET'.
'for' is the link.
Can you help me to learn more?
(Other definitions for security blanket that I've seen before include "comforter for child" , "information blackout" , "Embargo on information" , "comforting to the young?" , "It may help one sleep at night" .)