Serious listener at roost (7)
I believe the answer is:
earnest
'serious' is the definition.
(similar in meaning)
'listener at roost' is the wordplay.
'listener' becomes 'ear' (body part which hears or listens).
'at' says to put letters next to each other.
'roost' becomes 'nest' (I've seen this before).
'ear'+'nest'='EARNEST'
(Other definitions for earnest that I've seen before include "Sneer at (anag) -- serious" , "Art deserving" , "Sincere; determined" , "art making money?" , "Serious; pledge" .)