Agreement to have pet on lead when inside (9)
I believe the answer is:
concordat
'agreement' is the definition.
(I know that concordat is a type of written agreement)
'pet on lead when inside' is the wordplay.
'pet' becomes 'cat' (cats are often kept as pets).
'lead when' becomes 'cord' (I can't explain this - if you can you should believe this answer much more).
'inside' means one lot of letters goes inside another.
'on'+'cord'='oncord'
'cat' placed around 'oncord' is 'CONCORDAT'.
'to have' acts as a link.
Can you help me to learn more?
(Other definitions for concordat that I've seen before include "papal blessing" , "Signed agreement" , "pact" , "Agreement with Rome" .)