The decimated part (5)
I believe the answer is:
tenth
'part' is the definition.
(I've seen this before)
'the decimated' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'decimated' becomes 'enth' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'enth'='TENTH'
Can you help me to learn more?
(Other definitions for tenth that I've seen before include "0.1" , "Fraction in tithe" , "Decimal part?" , "after nine" , "(Decimal?) fraction" .)