Book European camps as well (7)
I believe the answer is:
besides
'as well' is the definition.
(thesaurus)
'book european camps' is the wordplay.
'book' becomes 'b'.
'european' becomes 'e' (abbreviation e.g. EU).
'camps' becomes 'sides' (I've seen this before).
'b'+'e'+'sides'='BESIDES'
(Other definitions for besides that I've seen before include "to boot" , "Additionally" , "Apart from" , "What's more" , "See bids (anag)" .)