Horrify a mate, eating prawn's head (5)
I believe the answer is:
appal
'horrify' is the definition.
(I know that horrify can be written as appal)
'a mate eating prawn's head' is the wordplay.
'mate' becomes 'pal' (both can mean a friend).
'eating' means one lot of letters goes inside another.
'head' suggests taking the first letters.
The initial letter of 'prawns' is 'p'.
'pal' placed around 'p' is 'ppal'.
'a'+'ppal'='APPAL'
(Other definitions for appal that I've seen before include "Put at rest (suspicion)" , "exhibiting dismay" , "Outrage" , "Greatly dismay" , "Greatly horrify" .)