Better pet run is designed (6)
I believe the answer is:
punter
'better' is the definition.
(synonyms)
'pet run is designed' is the wordplay.
'is designed' indicates an anagram (designed with the letters).
'pet'+'run'='petrun'
'petrun' anagrammed gives 'PUNTER'.
(Other definitions for punter that I've seen before include "Gambler; customer" , "A colloquial better or a regular client" , "Professional gambler; client (inf.)" , "Informally, a customer or someone who bets" , "One betting; 4's prey (slang)" .)