A number entering branch that's on the decline (6)
I believe the answer is:
waning
'on the decline' is the definition.
(I've seen this in another clue)
'a number entering branch' is the wordplay.
'number' becomes 'n' (n can represent any number).
'entering' indicates putting letters inside.
'branch' becomes 'wing'.
'a'+'n'='an'
'an' going inside 'wing' is 'WANING'.
'that's' acts as a link.
(Other definitions for waning that I've seen before include "(Of the moon) reducing in apparent size" , "(Of moon) decreasing" , "Gradually decreasing in size" , "Becoming weaker" , "Like the moon in last quarter" .)