Seriously premature, going round breeding ground (9)
I believe the answer is:
earnestly
'seriously' is the definition.
('earnestly' can be a synonym of 'seriously')
'premature going round breeding ground' is the wordplay.
'premature' becomes 'early' ('early' can be similar in meaning to 'premature').
'going round' means one lot of letters goes inside another.
'breeding ground' becomes 'nest' (I've seen this in other clues).
'early' placed around 'nest' is 'EARNESTLY'.
(Other definitions for earnestly that I've seen before include "Relent, say, in sincere and serious manner" , "In all seriousness" , "Sincerely and seriously" , "with heart and soul" , "Showing commitment" .)