Spur to save Britain (4)
I believe the answer is:
barb
'spur' is the definition.
(thesaurus)
'save britain' is the wordplay.
'save' becomes 'bar' (I've seen this before).
'britain' becomes 'B' (common abbreviation as in GB).
'bar'+'b'='BARB'
'to' is the link.
(Other definitions for barb that I've seen before include "nasty comment" , "Slight" , "Part of an arrow head" , "Backward projecting point of an arrowhead" , "A thorn or a gibe" .)