Serious organ needs home (7)
I believe the answer is:
earnest
'serious' is the definition.
('earnest' can be similar in meaning to 'serious')
'organ needs home' is the wordplay.
'organ' becomes 'ear' (ear is a kind of organ).
'needs' means one lot of letters go next to another.
'home' becomes 'nest' (I've seen this before).
'ear'+'nest'='EARNEST'
(Other definitions for earnest that I've seen before include "Eastern (anag.)" , "Art deserving" , "guarantee" , "Sneer at (anag) -- serious" , "art collector" .)