Suitable proportion for a lion-hearted rodent! (5)
I believe the answer is:
ratio
'rodent' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'suitable proportion for a lion-hearted' is the wordplay.
'suitable proportion for' becomes 'rat' (I am not sure about this - if you are sure you should believe this answer much more).
'hearted' means to look at the middle letters.
The middle letters of 'lion' are 'io'.
'rat'+'io'='RATIO'
Can you help me to learn more?
(Other definitions for ratio that I've seen before include "Quotient, proportion" , "E.g. two to one" , "Relationship between two numerical quantities" , "eg 1:3" , "One-to-one, maybe" .)