Gatecrashes popular, authentic society holding first of dances (8)
I believe the answer is:
intrudes
'gatecrashes' is the definition.
('intrude' can be a synonym of 'gatecrash')
'popular authentic society holding first of dances' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'authentic' becomes 'true' (I've seen this before).
'society' becomes 's' (common abbreviation in names of organisations).
'holding' means one lot of letters goes inside another.
'first of dances' becomes 'd' (1st letter of 'dances').
'true'+'s'='trues'
'trues' going around 'd' is 'trudes'.
'in'+'trudes'='INTRUDES'
(Other definitions for intrudes that I've seen before include "Interferes" , "Enters uninvited and unwelcome" , "Comes or joins in without being wanted" , "Imposes without welcome" .)