Shock a friend welcoming pressure (5)
I believe the answer is:
appal
'shock' is the definition.
('appal' can be a synonym of 'shock')
'a friend welcoming pressure' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend**).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'pressure' becomes 'p' (abbreviation).
'pal' going around 'p' is 'ppal'.
'a'+'ppal'='APPAL'
(Other definitions for appal that I've seen before include "expressed dismay" , "Shock, horrify" , "Greatly horrify" , "To shock, dismay" , "exhibiting dismay" .)