Ladies not finishing frivolous chatter? That's a hypothetical concept (5,3)
I believe the answer is:
ideal gas
'ladies not finishing frivolous chatter?' is the definition.
I can't tell whether this definition defines the answer.
'hypothetical concept' is the wordplay.
'hypothetical' becomes 'ideal' (I've seen this in another clue).
'concept' becomes 'gas' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ideal'+'gas'='IDEAL GAS'
'that's a' is the link.
Can you help me to learn more?
(Other definitions for ideal gas that I've seen before include "that's chemically perfect?" , "Perfectly-behaving substance in chemistry" , "Theoretical perfect substance" , "Hypothetical element" , "Theoretical perfect fluid" .)