Largely genuine lad in grounds (6)
I believe the answer is:
reason
'grounds' is the definition.
(thesaurus)
'largely genuine lad' is the wordplay.
'largely' means to remove the last letter.
'genuine' becomes 'real' ('real' is associated in meaning with 'genuine').
'lad' becomes 'son'.
'real' with its last letter taken away is 'rea'.
'rea'+'son'='REASON'
'in' is the link.
(Other definitions for reason that I've seen before include "Sanity and logic" , "Sanity of senora" , "Purpose" , "Motivation or cause" , "Logic of senora" .)