Uneven quality of empty devotion in poems on saint (7)
I believe the answer is:
oddness
'uneven quality' is the definition.
(odd is the opposite of even)
'of empty devotion in poems on saint' is the wordplay.
'of empty' means to remove the middle letters (I've seen 'emptied' mean this).
'in' means one lot of letters goes inside another.
'poems' becomes 'odes' (ode is a kind of poem).
'on' means one lot of letters go next to another.
'saint' becomes 's'.
'devotion' with its centre removed is 'dn'.
'dn' inserted inside 'odes' is 'oddnes'.
'oddnes'+'s'='ODDNESS'
(Other definitions for oddness that I've seen before include "a peculiarity of character" , "Curious state" , "Peculiar quality" , "Eccentricity" , "It isn't even" .)