One who starts to go down (7)
I believe the answer is:
founder
'one who' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'starts to go down' is the wordplay.
'starts' means to remove the last letter.
'to' becomes 'for'.
'go down' becomes 'under' (I've seen this before. I am not sure about the 'go' bit.).
'for' with its last letter taken off is 'fo'.
'fo'+'under'='FOUNDER'
Can you help me to learn more?
(Other definitions for founder that I've seen before include "One who establishes an institution" , "Sink, come to grief" , "One sets up firm" , "(Of ship) fill with water and sink" , "Sink like originator?" .)