Releases mysterious logs about extra-terrestrial (4,2)
I believe the answer is:
'releases' is the definition.
('let go' can be a synonym of 'release')
'mysterious logs about extra-terrestrial' is the wordplay.
'mysterious' is an anagram indicator.
'about' means one lot of letters goes inside another.
'extra terrestrial' becomes 'et' (abbreviation - like in the famous 1982 movie).
'logs' with letters rearranged gives 'lsgo'.
'lsgo' enclosing 'et' is 'LETS GO'.
(Other definitions for lets go that I've seen before include "Releases from one's grip" , "Liberates" , "fires" , "Order to leave" , "Come on then, we're off!" .)