Approximate time for filling area round building (7)
I believe the answer is:
rotunda
'round building' is the definition.
(architectural term)
'approximate time for filling area' is the wordplay.
'approximate' becomes 'round' (I've seen this in another clue).
'time' becomes 't' (abbreviation).
'for filling' is an insertion indicator (one set of letters goes inside another).
'area' becomes 'a' (maths abbreviation).
'round' placed around 't' is 'rotund'.
'rotund'+'a'='ROTUNDA'
(Other definitions for rotunda that I've seen before include "Round building or hall" , "Circular room or building" , "Round domed room" , "Italian Pantheon, for instance" , "domed hall" .)