First to authorise (7)
I believe the answer is:
initial
I believe this clue is a double definition.
'first' is the first definition.
('initial' can be similar in meaning to 'first')
'authorise' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for initial that I've seen before include "Word's first letter" , "First letter of a name" , "Letter - opening" , "First home, corgi, ultimately, wagging tail" , "Introductory" .)