Native restraining a cross British invader (5)
I believe the answer is:
saxon
'british invader' is the definition.
(the Saxons invaded Britain)
'native restraining a cross' is the wordplay.
'native' becomes 'son' (a 'son' of a particular place is a native of it).
'restraining' indicates putting letters inside.
'cross' becomes 'x' (X is cross-shaped).
'a'+'x'='ax'
'son' enclosing 'ax' is 'SAXON'.
(Other definitions for saxon that I've seen before include "Old German" , "Germanic conqueror of southern England" , "dating from the first millennium?" , "Member of Germanic tribe settled in southern England after the Romans" , "Early invader of southern England" .)