Old man climbing with little desire (8)
I believe the answer is:
appetite
'desire' is the definition.
(I know that appetite is a type of craving)
'old man climbing with little' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'with' means one lot of letters go next to another.
'little' becomes 'petite' (similar in meaning).
'pa' reversed gives 'ap'.
'ap'+'petite'='APPETITE'
(Other definitions for appetite that I've seen before include "A feeling of craving something" , "enthusiasm" , "'Strong desire, craving (8)'" , "Hunger; desire" , "Physical desire as for food" .)