Beat up at last! (6)
I believe the answer is:
larrup
'beat' is the definition.
(larruping is a kind of beating)
'up at last' is the wordplay.
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'last' becomes 'larr' (I am not sure about this - if you are sure you should believe this answer much more).
'up' after 'larr' is 'LARRUP'.
Can you help me to learn more?
(Other definitions for larrup that I've seen before include "Flog" , "Hit hard" .)