Degeneration of a jetty makes one white-faced (7)
I believe the answer is:
pierrot
'degeneration' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'jetty makes one white-faced' is the wordplay.
I cannot quite see how this works, but
'jetty' could be 'pier' (I've seen this before) and 'pier' is located in the answer.
The remaining letters 'rot' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'of a' acts as a link.
Can you help me to learn more?
(Other definitions for pierrot that I've seen before include "I report on French pantomime clown" , "Clown from French pantomime with white face and costume - tripe or what?" , "Clown from French panto - tripe or what" , "French panto clown with whitened face" , "Period; incantation" .)