Be back before Crosby's decline (6)
I believe the answer is:
ebbing
'decline' is the definition.
(ebbing is a kind of decline)
'be back before crosby's' is the wordplay.
'back' is a reversal indicator.
'before' says to put letters next to each other.
'crosby' becomes 'bing' (singer Bing Crosby).
'be' reversed gives 'eb'.
'eb'+'bing'='EBBING'
(Other definitions for ebbing that I've seen before include "Big Ben (anag)" , "Flowing out" , "Going out like a tide" , "in decline?" , "Gradually growing less" .)