Notice vicar rejected key part of speech (6)
I believe the answer is:
adverb
'part of speech' is the definition.
(adverb is a kind of part of speech)
'notice vicar rejected key' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'vicar' becomes 'rev'.
'rejected' says the letters should be written in reverse.
'key' becomes 'b' (eg B major, musical key).
'rev' reversed gives 'ver'.
'ad'+'ver'+'b'='ADVERB'
(Other definitions for adverb that I've seen before include "outside perhaps" , "Action word qualifier" , "Word as well, badly (gram.)" , "loudly, say" , "Verb qualifier" .)