Roundabout in Petersham bad for air? (10)
I believe the answer is:
atmosphere
'for air?' is the definition.
(synonyms)
'roundabout in petersham bad' is the wordplay.
'roundabout' becomes 'O' (resembles a roundabout).
'in' is an insertion indicator.
'bad' is an anagram indicator.
'petersham' anagrammed gives 'atmsphere'.
'o' going inside 'atmsphere' is 'ATMOSPHERE'.
(Other definitions for atmosphere that I've seen before include "We are always enveloped in this" , "Surrounding ambience" , "Feeling" , "Gas surrounding a planet" , "Air" .)