Alien returns the wrong cogs (5)
I believe the answer is:
teeth
'cogs' is the definition.
(I've seen this in another clue)
'alien returns the wrong' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'returns' is a reversal indicator.
'wrong' indicates anagramming the letters.
'et' written backwards gives 'te'.
'the' anagrammed gives 'eth'.
'te'+'eth'='TEETH'
(Other definitions for teeth that I've seen before include "Being in your face" , "genuine power" , "Genuine effectiveness" , "watchdog needs these" , "Saw or gearwheel items" .)