Briefly gain entry, on occasion to theatre (6)
I believe the answer is:
gaiety
'theatre' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'briefly gain entry on occasion' is the wordplay.
'briefly' means to remove the last letter.
'on occasion' means one should take alternating letters.
The alternate letters of 'entry' are 'ety'.
'gain' with its final letter taken away is 'gai'.
'gai'+'ety'='GAIETY'
'to' is the link.
Can you help me to learn more?
(Other definitions for gaiety that I've seen before include "Good humour" , "High spirits" , "Light-hearted state" , "Merriment, festive feeling" , "Merriment, revelry" .)