Shock when dad comes back with friend (5)
I believe the answer is:
appal
'shock' is the definition.
(I know that shock is a more specific form of appal)
'dad comes back with friend' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'comes back' says the letters should be written backwards.
'with' is a charade indicator (letters next to each other).
'friend' becomes 'pal' (pal can mean a friend).
'pa' backwards is 'ap'.
'ap'+'pal'='APPAL'
'when' acts as a link.
(Other definitions for appal that I've seen before include "expressed dismay" , "Shock, horrify" , "Put at rest (suspicion)" , "Scandalise" , "To shock, dismay" .)