Serious attention retreat needed (7)
I believe the answer is:
earnest
'serious' is the definition.
('earnest' can be similar in meaning to 'serious')
'attention retreat needed' is the wordplay.
'attention' becomes 'ear' (ear is a kind of attention).
'retreat' becomes 'nest' (nest is a kind of retreat).
'needed' says to put letters next to each other.
'ear'+'nest'='EARNEST'
(Other definitions for earnest that I've seen before include "Pledge - far from frivolous" , "Serious - down payment" , "promise" , "'Heartfelt, sincere (7)'" , "Fervent - down payment" .)