He's climbing mountain, conserving oxygen (3)
I believe the answer is:
tom
'he's' is the definition.
(I've seen this in another clue)
'climbing mountain conserving oxygen' is the wordplay.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'mountain' becomes 'mt'.
'conserving' means one lot of letters goes inside another.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'mt' placed around 'o' is 'mot'.
'mot' back-to-front is 'TOM'.
(Other definitions for tom that I've seen before include "Boy running off with pig" , "type pursuing queen" , "Jerry's cartoon antagonist" , "Boy - cat" , "Very small person" .)