Plant is favourite — any number I planted in middle of quad (7)
I believe the answer is:
petunia
'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' is an insertion indicator.
'middle of' indicates the central letters.
The middle of 'quad' is 'ua'.
'n'+'i'='ni'
'ni' placed into 'ua' is 'unia'.
'pet'+'unia'='PETUNIA'
'is' is the link.
(Other definitions for petunia that I've seen before include "Auntie foolishly made under pressure" , "Common garden 13 across" , "Flower - tuna pie (anag)" , "Plant related to tobacco" , "Common garden plant" .)