A crop of seed (5)
I believe the answer is:
acorn
'seed' is the definition.
(I've seen this before)
'a crop' is the wordplay.
'crop' becomes 'corn' (a farmer's crop of corn).
'a'+'corn'='ACORN'
'of' is the link.
(Other definitions for acorn that I've seen before include "Fruit-seed of the oak" , "Dirt on a liquid surface" , "form of seeding" , "mast" , "From which a "tall oak" may grow" .)