Grounds in manor house ultimately an issue (6)
I believe the answer is:
reason
'grounds' is the definition.
(I've seen this before)
'manor house ultimately an issue' is the wordplay.
'ultimately' suggests the final letters.
'an' becomes 'a'.
'issue' becomes 'son' (issue can mean someone's offspring).
The final letters of 'manor house' are 're'.
're'+'a'+'son'='REASON'
'in' is the link.
(Other definitions for reason that I've seen before include "Cause - logic" , "Defence" , "Explanation or motive" , "Incentive" , "Cause or motive" .)