Nowadays, an opening is always coming (6)
I believe the answer is:
advent
'always coming' is the definition.
'advent' can be an answer for 'coming' (advent is the arrival or coming of something). I am not certain of the 'always' bit.
'nowadays an opening' is the wordplay.
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'an opening' becomes 'vent' (a vent is an opening or outlet).
'ad'+'vent'='ADVENT'
'is' is the link.
Can you help me to learn more?
(Other definitions for advent that I've seen before include "Period in church calendar" , "Forty days preceding Christmas" , "Run-up to Christmas" , "Arrival of important person or event" , "Coming - occasion in Church calendar" .)