“Couple” has it in reverse (5)
I believe the answer is:
untie
'reverse' is the definition.
(I know that untie is a more specific form of the action the undo)
'couple has it' is the wordplay.
'couple' becomes 'tie' (both can mean to join).
'has' says to put letters next to each other.
'it' becomes 'un' (I can't justify this - if you can you should give a lot more credence to this answer).
'tie' put after 'un' is 'UNTIE'.
'in' is the link.
Can you help me to learn more?
(Other definitions for untie that I've seen before include "Let loose" , "get rid of bonds" , "Release bindings or shoelaces" , "Open a knot" , "Remove ropes, unfasten" .)