Seriously, it’s too soon to hold retreat (9)
I believe the answer is:
earnestly
'seriously' is the definition.
('earnestly' can be a synonym of 'seriously')
'too soon to hold retreat' is the wordplay.
'too soon' becomes 'early' (I've seen this before).
'to hold' is an insertion indicator.
'retreat' becomes 'nest' (nest is a kind of retreat).
'early' going around 'nest' is 'EARNESTLY'.
'it's' acts as a link.
(Other definitions for earnestly that I've seen before include "with gusto" , "Zealously" , "Showing serious intent" , "In all seriousness" , "with heart and soul" .)