Participant books to get into drug rave (7)
I believe the answer is:
entrant
'participant' is the definition.
(I've seen this before)
'books to get into drug rave' is the wordplay.
'books' becomes 'NT' (New Testament).
'to get into' means one lot of letters goes inside another.
'drug' becomes 'e' (E is another name for ecstasy).
'rave' becomes 'rant' (synonyms).
'e'+'rant'='erant'
'nt' inserted within 'erant' is 'ENTRANT'.
(Other definitions for entrant that I've seen before include "New participant" , "Someone coming in" , "New member or competitor" , "One taking part in a race or competition" , "One going in for exam" .)