Periodical maps out half the hot stuff (5)
I believe the answer is:
magma
'the hot stuff' is the definition.
(I've seen this before)
'periodical maps out half' is the wordplay.
'periodical' becomes 'mag' (abbreviation for magazine).
'maps out' becomes 'maps' ('map' can be a synonym of 'map out').
'half' means to take half the letters.
'maps' with half the letters taken is 'ma'.
'mag'+'ma'='MAGMA'
(Other definitions for magma that I've seen before include "Hot molten rock" , "Semi-fluid material within the earth's crust" , "Hot fluid rock" , "crust's content" , "Molten rock in the earth's crust" .)