Show yellow card to reserve (4)
I believe the answer is:
book
I believe this clue is a double definition.
'show yellow card' is the first definition.
(booking a footballer is showing them a yellow card)
'to reserve' is the second definition.
(to book something is to reserve it)
(Other definitions for book that I've seen before include "Tome, volume" , "Reserve - something to read" , "Volume - maybe thrown at offender" , "Make a reservation" , "Reading volume" .)