Dull period around two-thirds of holiday venue (8)
I believe the answer is:
tiresome
'dull' is the definition.
(similar in meaning)
'period around two-thirds of holiday venue' is the wordplay.
'period' becomes 'time period' (period of time).
'around' is an insertion indicator.
'two-thirds of holiday venue' becomes 'reso' (I can't justify this - if you can you should believe this answer much more).
'time' going around 'reso' is 'TIRESOME'.
Can you help me to learn more?
(Other definitions for tiresome that I've seen before include "irritating" , "Like a pain" , "Fatiguing, tedious" , "Is remote, being boring" , "Aggravating" .)