Military redirects molten rock north (5)
I believe the answer is:
naval
'military' is the definition.
(thesaurus)
'redirects molten rock north' is the wordplay.
'redirects' becomes 'n' (I can't explain this - if you can you should give a lot more credence to this answer).
'molten rock' becomes 'lava' (I've seen this before).
'north' is a reversal indicator (in a down clue, letters go up or 'northward').
'lava' back-to-front is 'aval'.
'n'+'aval'='NAVAL'
Can you help me to learn more?
(Other definitions for naval that I've seen before include "Relates to nautical defence force" , "Relating to warships" , "Relating to nautical defence forces" , "To do with the marine branch of an army" , "Relating to nautical defence force" .)