Energy used by soldier, possibly, climbing mountain (4)
I believe the answer is:
etna
'mountain' is the definition.
(Etna is an example)
'energy used by soldier possibly climbing' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'used by' says to put letters next to each other.
'soldier possibly' becomes 'ant' (as in army ants).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'ant' in reverse letter order is 'tna'.
'e'+'tna'='ETNA'
(Other definitions for etna that I've seen before include "Neat volcano overturned" , "Volcano in Italy" , "one blowing top?" , "Threatening mountain" , "hole in the crust" .)