Worn out tetra swimming in bottom of lake (8)
I believe the answer is:
battered
'worn' is the definition.
(similar in meaning)
'tetra swimming in bottom of lake' is the wordplay.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'in' means one lot of letters goes inside another.
'bottom of lake' becomes 'bed' (I can't explain this - if you can you should give a lot more credence to this answer).
'tetra' is an anagram of 'atter'.
'atter' going inside 'bed' is 'BATTERED'.
'out' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for battered that I've seen before include "Buffeted" , "Damaged by repeated blows" , "Like fritters" , "Beaten" , "Beaten-up, knocked about" .)