What a couple can do when, again, there's an R in the month (5)
I believe the answer is:
marry
'what a couple' is the definition.
(thesaurus)
'again there's an r in the month' is the wordplay.
'again' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'there's an r' becomes 'r'.
'in the' means one lot of letters goes inside another.
'month' becomes 'may' (May is an example).
'r'+'r'='rr'
'rr' inserted into 'may' is 'MARRY'.
'can do when' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for marry that I've seen before include "Get hitched" , "Well I never!" , "Tie the knot; why! (Shak.)" , "Gain a wife or husband" , "Join - get wed" .)