Article penned by second-century bishop, one undermining union (4)
I believe the answer is:
scab
'one undermining union' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'article penned by second-century bishop' is the wordplay.
'article' becomes 'a' (indefinite article).
'penned by' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'century' becomes 'c' (common abbreviation).
'bishop' becomes 'b' (abbreviation used in chess).
's'+'c'+'b'='scb'
'a' placed within 'scb' is 'SCAB'.
Can you help me to learn more?
(Other definitions for scab that I've seen before include "Derogatory term for a strike-breaker" , "Crust of congealed blood" , "Crusted sore on cut or wound" , "Blackleg (colloq.)" , "Strikebreaker; mange" .)