Very good at describing Alexander (5)
I believe the answer is:
great
'very' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'good at describing alexander' is the wordplay.
'good' becomes 'g' (abbreviation).
'describing' is an insertion indicator (describe can mean to form the outline of).
'alexander' becomes 're' (I can't justify this - if you can you should give a lot more credence to this answer).
'g'+'at'='gat'
'gat' enclosing 're' is 'GREAT'.
Can you help me to learn more?
(Other definitions for great that I've seen before include "cracking" , "Mum's dad's mum, for example" , "The Plough" , "Big, large" , "Grand" .)