Assume authority to book holiday (4,5)
I believe the answer is:
take leave
'book holiday' is the definition.
(as in taking leave from work)
'assume authority' is the wordplay.
'assume' becomes 'take' (synonyms).
'authority' becomes 'leave' (as in permission or authorisation to do something).
'take'+'leave'='TAKE LEAVE'
'to' is the link.
(Other definitions for take leave that I've seen before include "Say goodbye - go on holiday" , "part" , "Say goodbye - going on holiday from work?" , "To depart" .)