Plant is favourite — any number I planted in middle of quad (7)
I believe the answer is:
Here is my best explanation:
'plant' is the definition.
The answer is an example.
'favourite any number i planted in middle of quad' is the wordplay.
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'any number' becomes 'n' (I've seen this in other clues).
'planted in' indicates putting letters inside.
'middle of' means to look at the middle letters.
The central letters of 'quad' are 'ua'.
'ni' put inside 'ua' is 'unia'.
'is' is the link.
(Other definitions for petunia that I've seen before include "Plant related to tobacco", "Popular plant for baskets", "Bloomer", "Auntie foolishly made under pressure", "Garden plant, originally from S. America", "Common garden plant", "Flowering plant", "Flower - tuna pie (anag)", "Colourful" and "Plant cultivated for its white, pink, blue or purple funnel-shaped flowers".)