A suitable amount of decimation (5)
I believe the answer is:
tenth
'decimation' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'a suitable amount' is the wordplay.
'a suitable' becomes 'tent' (I can't justify this - if you can you should give a lot more credence to this answer).
'amount' becomes 'H' (H is an example).
'tent'+'h'='TENTH'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for tenth that I've seen before include "(Decimal?) fraction" , "0.1" , "Decimal part?" , "One behind ninth" , "Ordinal number" .)