Organ is in concluding part of performance (5)
I believe the answer is:
finis
'performance' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'organ is in concluding part' is the wordplay.
'organ' becomes 'f' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'concluding' says to put letters next to each other.
'part' is an anagram indicator (I've seen 'parts of' mean this).
'is'+'in'='isin'
'isin' is an anagram of 'inis'.
'f'+'inis'='FINIS'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for finis that I've seen before include "conclusion!" , "End (of book or film)" .)