Going back again to conceal a listening device (3)
I believe the answer is:
ear
'listening device' is the definition.
(I've seen this in another clue)
'going back again to conceal a' is the wordplay.
'going back' says the letters should be written backwards.
'again' becomes 're' (prefix meaning 'again').
'to conceal' indicates putting letters inside.
're' placed around 'a' is 'rae'.
'rae' written backwards gives 'EAR'.
(Other definitions for ear that I've seen before include "Musical appreciation" , "Shell-like (slang)" , "good sense of music" , "Musical ability" , "Van Gogh missing body part" .)