Figure mountain climbing, one occupying space (6)
I believe the answer is:
planet
'one occupying space' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'figure mountain climbing' is the wordplay.
'figure' becomes 'ten' (I've seen this before).
'mountain' becomes 'alp' (the Alps are a mountain range).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'ten'+'alp'='tenalp'
'tenalp' in reverse letter order is 'PLANET'.
Can you help me to learn more?
(Other definitions for planet that I've seen before include "Orbiting body in the heavens" , "Eg, Mars or Pluto" , "Venus, for example" , "Satellite of a sun" , "Heavenly (or earthly!) body" .)