Singers in church round one side of altar (5)
I believe the answer is:
choir
'singers' is the definition.
(group of singers)
'church round one side of altar' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'round' becomes 'o' (round can mean a round object).
'one' becomes 'I' (Roman numeral).
'side of' indicates one should take the final letters.
The last letter of 'altar' is 'r'.
'ch'+'o'+'i'+'r'='CHOIR'
'in' is the link.
(Other definitions for choir that I've seen before include "Vocal group" , "Body of singers" , "Vocalists in group" , "set of singers" , "warblers" .)