Corrective treatment in case of arson cheers police (7)
I believe the answer is:
antacid
'corrective treatment' is the definition.
(antacid corrects acidity levels)
'case of arson cheers police' is the wordplay.
'case of' suggests removing the centre (outsides of).
'cheers' becomes 'ta' (both can informally mean thank you).
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'arson' with its middle taken out is 'an'.
'an'+'ta'+'cid'='ANTACID'
'in' is the link.
(Other definitions for antacid that I've seen before include "Substance used to treat stomach upsert" , "Indigestion relief" , "calming agent" , "heartburn remedy" , "Indigestion cure" .)