Too advanced, therefore overwhelming learner (4)
I believe the answer is:
also
'too' is the definition.
(synonyms)
'advanced therefore overwhelming learner' is the wordplay.
'advanced' becomes 'a' (abbreviation).
'therefore' becomes 'so' ('so' can mean 'consequently' or 'therefore').
'overwhelming' means one lot of letters goes inside another.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'a'+'so'='aso'
'aso' placed around 'l' is 'ALSO'.
(Other definitions for also that I've seen before include "What's more" , "'Besides, moreover (4)'" , "Plus" , "Item" , "revolutionary a part for Yugoslavia" .)