A warning about that's put outside (6)
I believe the answer is:
threat
'a warning' is the definition.
(threat is a kind of warning)
'about that's put outside' is the wordplay.
'about' becomes 're' (regarding).
'put outside' indicates putting letters inside.
're' placed inside 'that' is 'THREAT'.
(Other definitions for threat that I've seen before include "Aggressive declaration" , "Impending danger" , "Hatter (anag)" , "Statement of intent to do harm" , "Possibility of danger" .)