Support book event (5)
I believe the answer is:
brace
'support' is the definition.
(brace is a kind of support)
'book event' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'event' becomes 'race' (race is a kind of event).
'b'+'race'='BRACE'
(Other definitions for brace that I've seen before include "Steady; pair" , "Stiffen; pair" , "Pair of game-birds" , "Pair - wood-boring tool" , "Strengthener; pair" .)