Defeat finally complete after reversal, lose it (5)
I believe the answer is:
erupt
'lose it' is the definition.
(I've seen this in another clue)
'defeat finally complete after reversal' is the wordplay.
'finally' indicates one should take the final letters.
'complete' becomes 'pure' (I can't explain this - if you can you should give a lot more credence to this answer).
'after' is a charade indicator (letters next to each other).
'reversal' shows that the letters should be reversed in order.
The last letter of 'defeat' is 't'.
't'+'pure'='tpure'
'tpure' in reverse letter order is 'ERUPT'.
Can you help me to learn more?
(Other definitions for erupt that I've seen before include "Break out suddenly and violently" , "Break out like Etna" , "Emit lava" , "go up" , "Explode" .)