Devout deserve good man holding evensong opening (7)
I believe the answer is:
earnest
'devout' is the definition.
('earnest' can be a synonym of 'devout')
'deserve good man holding evensong opening' is the wordplay.
'deserve' becomes 'earn' (I've seen this before).
'good man' becomes 'st'.
'holding' means one lot of letters goes inside another.
'opening' says to take the initial letters.
The initial letter of 'evensong' is 'e'.
'earn'+'st'='earnst'
'earnst' going around 'e' is 'EARNEST'.
(Other definitions for earnest that I've seen before include "Serious, intent" , "Sneer at (anag) -- serious" , "Fixed" , "Serious - advance" , "art collector" .)