Onset of decline, then Long Depression (5)
I believe the answer is:
ditch
'depression' is the definition.
(I have seen 'long depression' mean 'ditch' so perhaps 'depression' could also mean 'ditch')
'onset of decline then long' is the wordplay.
'onset of' indicates taking the first letters.
'then' says to put letters next to each other.
'long' becomes 'itch' (to long or itch for something is to desire it).
The first letter of 'decline' is 'd'.
'd'+'itch'='DITCH'
(Other definitions for ditch that I've seen before include "Land in water" , "Drainage trench" , "Excavated channel" , "Get rid of; dug channel" , "Drainage channel" .)