Said to be concerned with Barnaby? Make merry! (7)
I believe the answer is:
rejoice
'make merry' is the definition.
(I've seen this before)
'said to be concerned with' is the wordplay.
'said' becomes 'joice' (I can't explain this - if you can you should believe this answer much more).
'to' means one lot of letters go next to another (I've seen this in other clues).
'be concerned with' becomes 're' (re can mean regarding, concerning).
'joice' after 're' is 'REJOICE'.
'barnaby?' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for rejoice that I've seen before include "Express delight" , "Celebrate" , "Exult, show delight" , "Be exuberant" , "Express great happiness" .)