Lionesses, in their element, top this! (4,5)
I believe the answer is:
food chain
'top this' is the definition.
The definition suggests a singular noun which matches the answer.
'lionesses in their element' is the wordplay.
I cannot really see how this works, but
'in' is found in the answer.
'element' could be 'o' (O is an example) and 'o' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for food chain that I've seen before include "hierarchy" , "Natural order of consumption" , "string of sausages?" .)