Mountain ascended in spirit, you've done well (4,4)
I believe the answer is:
fair play
'you've done well' is the definition.
'mountain ascended in spirit' is the wordplay.
'mountain' becomes 'alp' (the Alps are a mountain range).
'ascended' says the letters should be written backwards (I've seen 'ascending' mean this).
'in' means one lot of letters goes inside another.
'spirit' becomes 'fairy' ().
'alp' reversed gives 'pla'.
'pla' going into 'fairy' is 'FAIR PLAY'.
(Other definitions for fair play that I've seen before include "Equitable or reasonable treatment" , "Just treatment of others" , "respect for the rules" , "Equal treatment" , "Equitable treatment or behaviour" .)