Favourite college next to a plant (7)
I believe the answer is:
petunia
'plant' is the definition.
(petunia is a kind of plant)
'favourite college next to a' is the wordplay.
'favourite' becomes 'pet' (synonyms).
'college' becomes 'uni' (short for university).
'next to' means one lot of letters go next to another.
'pet'+'uni'+'a'='PETUNIA'
(Other definitions for petunia that I've seen before include "Popular plant for baskets" , "Garden plant with large funnel-shaped flower" , "Flowering plant" , "Common garden 13 across" , "Plant cultivated for its white, pink, blue or purple funnel-shaped flowers" .)