Society leaving Waugh’s country house without good fortification (10)
I believe the answer is:
bridgehead
'fortification' is the definition.
(I know that bridgehead is a type of fortification)
'society leaving waugh's country house without good' is the wordplay.
'society' becomes 's' (common abbreviation in names of organisations).
'leaving' is a deletion indicator.
'waugh's country house' becomes 'Brideshead' (Brideshead Revisited is a novel by Evelyn Waugh).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'good' becomes 'g' (abbreviation).
'brideshead' with 's' taken out is 'bridehead'.
'bridehead' placed around 'g' is 'BRIDGEHEAD'.
(Other definitions for bridgehead that I've seen before include "Secured position from which to advance" , "Forward position seized by attacking troops" , "defensive position" , "Advantageous position" , "Advanced position secured in enemy territory" .)