Nowadays, I must work outside arena (7)
I believe the answer is:
stadium
'arena' is the definition.
(I know that arena can be written as stadium)
'nowadays i must work outside' is the wordplay.
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'work' indicates an anagram.
'outside' is an insertion indicator.
'must' with letters rearranged gives 'stum'.
'ad'+'i'='adi'
'adi' placed within 'stum' is 'STADIUM'.
(Other definitions for stadium that I've seen before include "Admit us (anag) - sports ground" , "Venue for sporting events, concerts etc" , "Sports ground as for athletics" , "A major sports ground" , "bowl?" .)