Thoroughly beat rather drab freshman, in essence (6)

I believe the answer is:

thrash

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'thoroughly beat' is the definition.
'thrash' can be an answer for 'beat' (I've seen this in another clue). I am not sure about the 'thoroughly' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for thrash that I've seen before include "Flog; party" , "Beat with stick or whip" , "Beat; party (colloq.)" , "Beat hard" , "Trounce, beat soundly" .)

I've seen this clue in The Irish Times.

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