Son leaves Waugh’s house, skirting key part of fortification (10)
I believe the answer is:
bridgehead
'part of fortification' is the definition.
(fortification protecting a bridge)
'son leaves waugh's house skirting key' is the wordplay.
'son' becomes 's'.
'leaves' indicates named letters should be taken away.
'waugh's house' becomes 'brideshead' (house in Evelyn Waugh's novel Brideshead Revisited).
'skirting' indicates putting letters inside.
'key' becomes 'g' (I've seen this in other clues).
'brideshead' with 's' taken out is 'bridehead'.
'bridehead' enclosing 'g' is 'BRIDGEHEAD'.
(Other definitions for bridgehead that I've seen before include "defensive position" , "Strong position secured inside enemy territory" , "Advantageous position" , "Forward position seized by attacking troops" , "Advanced position secured in enemy territory" .)