Travel companion shows reserve following girl in uniform (9)

I believe the answer is:

guidebook

'travel companion' is the definition.
(a traveller might use a guidebook)

'reserve following girl in uniform' is the wordplay.
'reserve' becomes 'book' (synonyms).
'following' says to put letters next to each other.
'girl in uniform' becomes 'guide' (as in the youth group Girl Guides).
'book' after 'guide' is 'GUIDEBOOK'.

'shows' is the link.

(Another definition for guidebook that I've seen is " Source of information for tourists".)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.