Casual workers conserving extremely large places of worship (7)
I believe the answer is:
temples
'places of worship' is the definition.
(I've seen this in another clue)
'casual workers conserving extremely large' is the wordplay.
'casual workers' becomes 'temps' (temporary workers).
'conserving' means one lot of letters goes inside another.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'large' with its middle removed is 'le'.
'temps' going around 'le' is 'TEMPLES'.
(Other definitions for temples that I've seen before include "shrines?" , "Buildings for the workshop of gods" , "Houses of worship" , "Sides of head or religious buildings" , "Cerebral" .)