Fanatical invasion involving British (5)
I believe the answer is:
rabid
'fanatical' is the definition.
(I know that fanatical can be written as rabid)
'invasion involving british' is the wordplay.
'invasion' becomes 'raid' (I've seen this in another clue).
'involving' means one lot of letters goes inside another.
'british' becomes 'b' (abbreviation e.g. in BBC).
'raid' enclosing 'b' is 'RABID'.
(Other definitions for rabid that I've seen before include "Mad like deranged dog" , "Violent; hydrophobic" , "Mad; raging" , "Burning" , "Furious like mad dog?" .)