Devoutness lacking core element is source of disappointment (4)
I believe the answer is:
pity
'source of disappointment' is the definition.
'devoutness lacking core element' is the wordplay.
'devoutness' becomes 'piety' (I've seen this before).
'lacking core element' says to hollow out the word (remove centre letters) (the core is the middle of the word).
'piety' with its centre taken out is 'PITY'.
'is' is the link.
(Other definitions for pity that I've seen before include "true compassion?" , "Have sympathetic feelings toward" , "unfortunate situation" , "Sympathy for the suffering of another" , "Feel sorrow for" .)