Notice minister flipped on 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 minister flipped on key' is the wordplay.
'notice' becomes 'ad' (short for advert).
'minister' becomes 'rev' (short for reverend).
'flipped' is a reversal indicator.
'on' is a charade indicator (letters next to each other).
'key' becomes 'b' (eg B major, musical key).
'rev' in reverse letter order is 'ver'.
'ad'+'ver'+'b'='ADVERB'
(Other definitions for adverb that I've seen before include "Verb modifier" , "Author of The Thirty-Nine Steps" , "outside perhaps" , "Word to qualify an action" , "Term of modification" .)