Make merry: go further in mountain valley (7)
I believe the answer is:
gladden
'make merry' is the definition.
(to gladden someone is to make them merry or happy)
'go further in mountain valley' is the wordplay.
'go further' becomes 'add' (I've seen this before).
'in' is an insertion indicator.
'mountain valley' becomes 'glen' (I've seen this in another clue).
'add' put into 'glen' is 'GLADDEN'.
(Other definitions for gladden that I've seen before include "please" , "Make pleased or happy" , "Make joyful" , "raise spirits" , "Delight" .)