Hardly a fast grower (7)
I believe the answer is:
creeper
'grower' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'hardly a' is the wordplay.
'hardly' becomes 'cree' (I can't explain this - if you can you should believe this answer much more).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'cree'+'per'='CREEPER'
'fast' acts as a link.
Can you help me to learn more?
(Other definitions for creeper that I've seen before include "Say, Virginia" , "Ivy or Virginia, for example" , "Variety of plant" , "Reptile" , "low-lying plant" .)