Masterly advert for a new pet (5)
I believe the answer is:
adept
'masterly' is the definition.
(thesaurus)
'advert for a new pet' is the wordplay.
'advert' becomes 'ad' (synonyms).
'for' is a charade indicator (letters next to each other).
'a new' indicates anagramming the letters.
'pet' with letters rearranged gives 'ept'.
'ad'+'ept'='ADEPT'
(Other definitions for adept that I've seen before include "Handy" , "efficient" , "Skilful and adroit" , "Highly proficient" , "skilled performer" .)