Conclusion somewhat lacking heart again! (6)
I believe the answer is:
encore
'again' is the definition.
(I've seen this before)
'conclusion somewhat lacking heart' is the wordplay.
'conclusion' becomes 'end' (end is a kind of conclusion).
'somewhat lacking' means to remove the last letter (most but not all of the word).
'heart' becomes 'core' (synonyms).
'end' with its final letter taken away is 'en'.
'en'+'core'='ENCORE'
(Other definitions for encore that I've seen before include "Request to sing it again" , "Bonus" , "Cry from the audience for more" , "Please play some more!" , "Repeat performance, please" .)