French retaining old castles in perpetuity (3,5)
I believe the answer is:
for keeps
'in perpetuity' is the definition.
Both the answer and definition are adverbs. Maybe they are linked in a way I don't understand?
'french retaining old castles' is the wordplay.
'french' becomes 'fr' (abbreviation).
'retaining' is an insertion indicator (inserted letters are held or retained).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'castles' becomes 'keeps' (I can't explain this - if you can you should give a lot more credence to this answer).
'fr' going around 'o' is 'for'.
'for'+'keeps'='FOR KEEPS'
Can you help me to learn more?
(Other definitions for for keeps that I've seen before include "indefinitely" , "with serious intent" , "Always" , "Permanently (colloq.)" , "in favour of strongholds" .)