Planet core's hot all the way down (5)
I believe the answer is:
earth
'planet' is the definition.
(Earth is an example)
'all the way down' is the wordplay.
'all' becomes 'ear' (I can't justify this - if you can you should believe this answer much more).
'way' becomes 'e' (Compass direction - East).
'down' suggests deleting specific letters (to be down something is to be missing it).
'the' with 'e' taken away is 'th'.
'ear'+'th'='EARTH'
'core's hot' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for earth that I've seen before include "Badger's burrow" , "See 12" , "Soil; electrical ground" , "Familiar example of 1 Across" , "costs so much?" .)