One born on, that is to say, an encrustation (4)
I believe the answer is:
scab
'say an encrustation' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'one born on that is' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'born' becomes 'b' (abbreviation next to the year of someone's birth).
'on' is a charade indicator (letters next to each other).
'that is' becomes 'sc' (abbreviation for Latin scilicet meaning 'namely').
'a'+'b'='ab'
'ab' after 'sc' is 'SCAB'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for scab that I've seen before include "Encrustation for the strikebreaker" , "Blackleg - sign of injury" , "Someone who works during a strike" , "sign of wound healing" , "cover" .)