Molten rock in periodical almost duplicated? (5)
I believe the answer is:
magma
'molten rock' is the definition.
(molten rock from a volcano)
'in periodical almost duplicated?' is the wordplay.
'in' is an insertion indicator.
'periodical' becomes 'mag' (short for magazine).
'almost' means to remove the last letter (most of the word but not all of it).
'duplicated?' becomes 'agm' (I can't justify this - if you can you should give a lot more credence to this answer).
'mag' with its final letter taken away is 'ma'.
'ma' going around 'agm' is 'MAGMA'.
Can you help me to learn more?
(Other definitions for magma that I've seen before include "That's hot!" , "Lava" , "Hot molten rock" , "Semi-fluid material within the earth's crust" , "Molten rock in the earth's crust" .)