Nice crop developed by American astronomer (10)
I believe the answer is:
copernicus
'astronomer' is the definition.
(I've seen this before)
'nice crop developed by american' is the wordplay.
'developed' indicates an anagram.
'by' says to put letters next to each other.
'american' becomes 'US' (abbreviation for United States).
'nice'+'crop'='nicecrop'
'nicecrop' anagrammed gives 'copernic'.
'copernic'+'us'='COPERNICUS'
(Other definitions for copernicus that I've seen before include "Polish scholar" , "Famous Prussian" , "16th century astronomer" , "Polish astronomer d. 1543" , "He said we move in a ring" .)