Observe Harding, latterly in the ascendant, prosper (3,2)
I believe the answer is:
get on
'prosper' is the definition.
(I've seen this before)
'observe harding latterly in the ascendant' is the wordplay.
'observe' becomes 'note' (synonyms).
'harding latterly' becomes 'g' (I can't explain this - if you can you should give a lot more credence to this answer).
'in the ascendant' is a reversal indicator.
'note'+'g'='noteg'
'noteg' in reverse letter order is 'GET ON'.
Can you help me to learn more?
(Other definitions for get on that I've seen before include "Proceed; prosper" , "Make progress or mount up" , "Make progress; be friends" , "Board; establish good relations" , "Make progress or climb aboard" .)