A popular plant for a racecourse (7)
I believe the answer is:
aintree
'racecourse' is the definition.
(I've seen this before)
'a popular plant for a' is the wordplay.
'a popular' becomes 'in'.
'plant' becomes 'tree' (tree is a kind of plant**).
'for' means one lot of letters go next to another (I've seen this in other clues).
'in'+'tree'='intree'
'intree' put after 'a' is 'AINTREE'.
(Other definitions for aintree that I've seen before include "Here they run" , "Liverpool racecourse" , "Trainee in Grand National venue" , "National racecourse" , "Liverpool's racecourse" .)