Put under lock and key by Frenchmen going to old city in Ireland (6)
I believe the answer is:
immure
'put under lock and key' is the definition.
I can't tell whether this defines the answer.
'frenchmen going to old city in ireland' is the wordplay.
'frenchmen' becomes 'mm' (two M's - M is short for Monsieur).
'going to' means one lot of letters go next to another.
'old city' becomes 'ur' (ancient city state).
'in' is an insertion indicator.
'ireland' becomes 'ie' (two letter country code).
'mm'+'ur'='mmur'
'mmur' put within 'ie' is 'IMMURE'.
'by' is the link.
Can you help me to learn more?
(Other definitions for immure that I've seen before include "Enclose within walls, incarcerate" , "Confine; wall in" , "Imprison within walls" , "Confine in a closed space" , "Sketches" .)