Grounds are churned up by boy (6)
I believe the answer is:
reason
'grounds' is the definition.
(thesaurus)
'churned up by boy' is the wordplay.
'churned up' becomes 'rea' (I can't justify this - if you can you should believe this answer much more).
'by' is a charade indicator (letters next to each other).
'boy' becomes 'son' ('son' can be a synonym of 'boy').
'rea'+'son'='REASON'
'are' is the link.
Can you help me to learn more?
(Other definitions for reason that I've seen before include "Justification; sanity" , "Cause; mind's power" , "Reach logical conclusions" , "Justification needed" , "Cause to perform" .)