Clever advert for a new pet (5)
I believe the answer is:
adept
'clever' is the definition.
(adept can mean clever or expert)
'advert for a new pet' is the wordplay.
'advert' becomes 'ad' (synonyms).
'for' means one lot of letters go next to another.
'a new' indicates an anagram.
'pet' with letters rearranged gives 'ept'.
'ad'+'ept'='ADEPT'
(Other definitions for adept that I've seen before include "clever" , "efficient" , "Dexterous" , "Very skilled person" , "Proficient, skilled" .)