Mob apt to exult at the grand finale (5)
I believe the answer is:
crowd
'mob' is the definition.
(thesaurus)
'at the grand finale' is the wordplay.
'at' becomes 'crow' (I can't justify this - if you can you should give a lot more credence to this answer).
'finale' suggests the final letters.
The last letter of 'grand' is 'd'.
'crow'+'d'='CROWD'
'apt to exult' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for crowd that I've seen before include "Spectators" , "Mob of people" , "Horde" , "Masses" , "Army" .)