Reason cocoa is drunk before start of nightfall (8)
I believe the answer is:
occasion
'reason' is the definition.
(occasion can mean the reason for something)
'cocoa is drunk before start of nightfall' is the wordplay.
'drunk' indicates an anagram.
'before' means one lot of letters go next to another.
'start of' indicates taking the first letters.
The initial letter of 'nightfall' is 'n'.
'cocoa'+'is'='cocoais'
'cocoais' with letters rearranged gives 'occasio'.
'occasio'+'n'='OCCASION'
(Other definitions for occasion that I've seen before include "Need" , "Provoke" , "call" , "Time of a particular event" , "Social event or occurrence" .)