Drinks excessively at first, leading to fall (5)
I believe the answer is:
lapse
'fall' is the definition.
(I have seen 'Moral fall ' mean 'lapse' so perhaps 'fall' could also mean 'lapse')
'drinks excessively at first' is the wordplay.
'drinks' becomes 'laps' (lapping is a kind of drinking).
'at first' says to take the initial letters.
The initial letter of 'excessively' is 'e'.
'laps'+'e'='LAPSE'
'leading to' is the link.
(Other definitions for lapse that I've seen before include "Slip; sounds like circuits" , "Loss (in concentration, say?)" , "A slight error or a passage of time" , "Fall back" , "Error; oversight" .)