Trap John on the way back (3)
I believe the answer is:
gob
'trap' is the definition.
(I know that trap can be written as gob)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for gob that I've seen before include "A slangy mouth" , "Mouth (slang)" , "Clot of a slimy substance found in bog" .)