Angry son joined forces (7,2)
I believe the answer is:
steamed up
'angry' is the definition.
'son joined forces' is the wordplay.
'son' becomes 's'.
'joined forces' becomes 'teamedup'.
's'+'teamedup'='STEAMED UP'
(Other definitions for steamed up that I've seen before include "Mad" , "Indignant (colloq.)" , "in a stew" , "window being so opaque" , "Annoyed" .)