Dad taken aback by friend's dismay (5)
I believe the answer is:
appal
'dismay' is the definition.
(synonyms)
'dad taken aback by friend's' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'taken aback' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'friend' becomes 'pal' (pal can mean a friend).
'pa' reversed gives 'ap'.
'ap'+'pal'='APPAL'
(Other definitions for appal that I've seen before include "Put at rest (suspicion)" , "Cause to be unpleasantly surprised" , "Fill with horror" , "expressed dismay" , "Scandalise" .)