Father upset over mate's dismay (5)
I believe the answer is:
appal
'dismay' is the definition.
('appal' can be a synonym of 'dismay')
'father upset over mate's' is the wordplay.
'father' becomes 'pa' (pa is a kind of father).
'upset' is a reversal indicator.
'over' is a charade indicator (letters next to each other).
'mate' becomes 'pal' (I've seen this before**).
'pa' written backwards gives 'ap'.
'ap'+'pal'='APPAL'
(Other definitions for appal that I've seen before include "Put at rest (suspicion)" , "Dismay, horrify" , "Greatly dismay" , "Disgust" , "Shock, horrify" .)