Warning: Model has broken heart (6)
I believe the answer is:
threat
'warning' is the definition.
(threat is a kind of warning)
'model has broken heart' is the wordplay.
'model' becomes 't' (Ford Model T car).
'has' means one lot of letters go next to another.
'broken' indicates anagramming the letters.
'heart' is an anagram of 'hreat'.
't'+'hreat'='THREAT'
(Other definitions for threat that I've seen before include "Warning of intent to harm" , "The rat became a menace" , "Statement of harmful intent" , "Aggressive declaration" , "Expression of intention to harm" .)