A capital crop (7)
I believe the answer is:
haircut
'a capital crop' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'capital crop' is the wordplay.
'capital' becomes 'hair' (I can't justify this - if you can you should believe this answer much more).
'crop' becomes 'cut'.
'hair'+'cut'='HAIRCUT'
Can you help me to learn more?
(Other definitions for haircut that I've seen before include "Style of coiffure" , "DA, for example" , "Bob for one" , "Salon service" , "Bob, perhaps" .)