Assured Great Dane managed to secure 3rd in Crufts (10)
I believe the answer is:
guaranteed
'assured' is the definition.
(guaranteeing is a kind of assuring)
'secure 3rd in crufts' is the wordplay.
'secure' becomes 'guarantee' (synonyms).
'3rd in crufts' becomes 'd' (I am not sure about this - if you are sure you should believe this answer much more).
'guarantee'+'d'='GUARANTEED'
'great dane managed to' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for guaranteed that I've seen before include "Warranted" , "Supported" , "Promised" , "protected" , "swore" .)