Amount of heat concealed by weathermen (5)
I believe the answer is:
therm
'amount of heat' is the definition.
(I've seen this before)
'concealed by weathermen' is the wordplay.
'concealed by' says the answer is hidden in the clue.
'THERM' is hidden within 'weathermen'.
(Other definitions for therm that I've seen before include "it measures heat" , "Amount of heat" , "A hot unit" , "Unit of heat and gas measure" , "Unit of heat as in gas supply" .)