Invader was brave enough, having lost a number (7)
I believe the answer is:
hundred
'number' is the definition.
(hundred is a kind of number)
'invader was brave enough having lost a' is the wordplay.
'invader' becomes 'hun' (I have seen 'Ancient invader ' mean 'hun' so perhaps 'invader' could also mean 'hun').
'was brave enough' becomes 'dared' (I've seen this before).
'having lost' is a deletion indicator.
'dared' with 'a' taken out is 'dred'.
'hun'+'dred'='HUNDRED'
(Other definitions for hundred that I've seen before include "cricketing achievement" , "Subdivision of shire; ton" , "Former division of county" , "Five score" , "English county division" .)