Benefactor grabs alien, but it goes off (9)
I believe the answer is:
detonator
'it goes off' is the definition.
The definition suggests a singular noun which matches the answer.
'benefactor grabs alien' is the wordplay.
'benefactor' becomes 'donator' (I can't justify this - if you can you should give a lot more credence to this answer).
'grabs' means one lot of letters goes inside another.
'alien' becomes 'et' (extra terrestrial).
'donator' going around 'et' is 'DETONATOR'.
'but' is the link.
Can you help me to learn more?
(Other definitions for detonator that I've seen before include "Part of bomb" , "That has explosive effect" , "means to cause explosion" , "One sets off bang" , "this will cause an outburst" .)