A bit of heat delineated by weather map (5)
I believe the answer is:
therm
'a bit' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'heat delineated by weather map' is the wordplay.
'THERM' is hidden within the letters.
But, I am not sure how this is indicated.
'of' is the link.
Can you help me to learn more?
(Other definitions for therm that I've seen before include "Unit of heat as in gas supply" , "Former gas supply unit" , "Unit of heat used in the UK as the basis of gas bills" , "A hot unit" , "Amount of heat" .)