New pet diet, dream, planned in advance (12)
I believe the answer is:
premeditated
'planned in advance' is the definition.
(I've seen this before)
'new pet diet dream' is the wordplay.
'new' indicates anagramming the letters.
'pet'+'diet'+'dream'='petdietdream'
'petdietdream' is an anagram of 'PREMEDITATED'.
(Other definitions for premeditated that I've seen before include "Considered" , "Planned beforehand" , "Thought out beforehand" , "Planned in advance" .)