Unexpectedly, Tom ends fixed allowance, causing protest (13)
I believe the answer is:
demonstration
'protest' is the definition.
(I've seen this before)
'unexpectedly tom ends fixed allowance' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'fixed allowance' becomes 'ration' (I've seen this before).
'tom'+'ends'='tomends'
'tomends' is an anagram of 'demonst'.
'demonst'+'ration'='DEMONSTRATION'
'causing' acts as a link.
(Other definitions for demonstration that I've seen before include "March; proof" , "Protest march" , "Display showing how something works" , "It's to voice protest" , "Protest march/meeting" .)