Trounced? Great achievement indeed! (8)
I believe the answer is:
defeated
'trounced?' is the definition.
('defeat' can be a synonym of 'trounce')
'great achievement indeed' is the wordplay.
I cannot quite see how this works, but
'achievement' could be 'feat' (feat is a kind of achievement) and 'feat' is found in the answer.
The remaining letters 'deed' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for defeated that I've seen before include "thwarted" , "Vanquished" , "foiled" , "Beat; beaten" , "Did cream" .)