Having been got at inside, is defeated (6)
I believe the answer is:
beaten
'is defeated' is the definition.
(similar in meaning)
'having been got at' is the wordplay.
I cannot really understand how this works, but
'got' could be 'eaten' (I've seen this before) and 'eaten' is present in the answer.
A single letter 'b' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'inside' is the link.
Can you help me to learn more?
(Other definitions for beaten that I've seen before include "Defeated or mixed vigorously" , "Trounced" , "Defeated or overcome" , "Struck; defeated" , "Failed to win - punished" .)