Father guarding old castles on a permanent basis (3,5)
I believe the answer is:
for keeps
'on a permanent basis' is the definition.
I can't judge whether this definition defines the answer.
'father guarding old castles' is the wordplay.
'father' becomes 'fr' (abbreviation as in a priest's title).
'guarding' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'castles' becomes 'keeps' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fr' enclosing '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 "Always" , "in favour of strongholds" , "indefinitely" , "Permanently (colloq.)" , "with serious intent" .)