Dominoes beginning to fall for sap (5)
I believe the answer is:
drain
'for sap' is the definition.
(synonyms)
'dominoes beginning to fall' is the wordplay.
'beginning to' suggests taking the first letters.
'fall' becomes 'rain' (raining is a kind of falling).
The initial letter of 'dominoes' is 'd'.
'd'+'rain'='DRAIN'
(Other definitions for drain that I've seen before include "Waste pipe" , "Waste channel/pipe" , "Nadir (anag.)" , "Something using up resources" , "Drink dry" .)