Mix-up in tax claim is something of a disappointment (10)
I believe the answer is:
anticlimax
'a disappointment' is the definition.
(I've seen this before)
'mix-up in tax claim' is the wordplay.
'mix-up' is an anagram indicator.
'in'+'tax'+'claim'='intaxclaim'
'intaxclaim' with letters rearranged gives 'ANTICLIMAX'.
'is something of' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for anticlimax that I've seen before include "Damp squib" , "disappointment" , "disappointing outcome" , "Disappointing result" , "Let-down after excitement" .)