Lots of seats here are not available (5)
I believe the answer is:
arena
'lots of seats here' is the definition.
(an arena contains many seats)
'are not available' is the wordplay.
'not available' becomes 'na' (abbreviation for not available).
'are'+'na'='ARENA'
(Other definitions for arena that I've seen before include "Field of conflict" , "Ring; stadium" , "Place of combat" , "Area surrounded by seating, for sports, etc." , "Sporting venue enclosed by seats" .)