Before PM starts, a vitality that's remarkable (7)
I believe the answer is:
amazing
'remarkable' is the definition.
(thesaurus)
'before pm starts a vitality' is the wordplay.
'before' is a charade indicator (letters next to each other).
'pm' becomes 'may' (Theresa May).
'starts' means to remove the last letter.
'vitality' becomes 'zing' (I've seen this before**).
'may' with its last letter taken away is 'ma'.
'ma' after 'a' is 'ama'.
'ama'+'zing'='AMAZING'
'that's' is the link.
(Other definitions for amazing that I've seen before include "Astonishing, remarkable" , "Fantastic" , "Excellent; surprising" , "Very impressive" , "Gobsmackingly great!" .)