Strasbourg's ready with external, dubious rationale (7)
I believe the answer is:
pretext
'rationale' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for pretext that I've seen before include "False reason, guise" , "Excuse for an action" , "Fictitious reason in order to conceal the real one" , "Specious excuse" , "Justification, excuse" .)