Goes back over it again and finds extremely rare little fragments (8)
I believe the answer is:
retraces
'goes back over' is the definition.
(as in retracing your steps)
'rare little fragments' is the wordplay.
'rare' becomes 'red' (as in a red steak).
'little' means to remove the last letter (the word is not quite complete).
'fragments' becomes 'traces' (I can't justify this - if you can you should give a lot more credence to this answer).
'red' with its final letter removed is 're'.
're'+'traces'='RETRACES'
'it again and finds extremely' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for retraces that I've seen before include "Goes back over again" , "Follows back from terraces" .)