Bond is on alert breaking in! (12)
I believe the answer is:
relationship
'bond' is the definition.
(I have seen 'Emotional bond ' mean 'relationship' so perhaps 'bond' could also mean 'relationship')
'is on alert breaking in' is the wordplay.
'breaking' indicates anagramming the letters.
'in' becomes 'hip' (both can mean fashionable**).
'is'+'on'+'alert'='isonalert'
'isonalert' with letters rearranged gives 'relations'.
'relations'+'hip'='RELATIONSHIP'
(Other definitions for relationship that I've seen before include "emotional tie-up" , "Linkage; affair" , "Connection, bond" , "Family connection" , "Romance" .)