Part of speech raved about Britain (6)
I believe the answer is:
adverb
'part of speech' is the definition.
(adverb is a kind of part of speech)
'raved about britain' is the wordplay.
'about' is an anagram indicator.
'britain' becomes 'B' (common abbreviation as in GB).
'raved' anagrammed gives 'adver'.
'adver'+'b'='ADVERB'
(Other definitions for adverb that I've seen before include "Word modifying adjective" , "Perhaps stupidly" , "Word to qualify an action" , "-ly word" , "Verb modifier" .)