Terrible noise made by alien undergoing torture (6)
I believe the answer is:
Here is my best explanation:
'terrible noise' is the definition.
'racket' can be an answer for 'noise' (racket is a kind of noise). I am not sure about the 'terrible' bit.
'by alien undergoing torture' is the wordplay.
'by' means one lot of letters go next to another.
'alien' becomes 'et' (extra terrestrial).
'undergoing torture' becomes 'rack' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'et' put after 'rack' is 'RACKET'.
'made' is the link.
(Other definitions for racket that I've seen before include "See 1", "Sports equipment", "Infernal row", "Dodgy business", "Lot of noise or bat for game (6)", "Din, clamour", "Din - tennis bat", "fiddle", "Din; snowshoe" and "Illegal scheme".)