Second one is included in the notice (3)
I believe the answer is:
aid
'second' is the definition.
(thesaurus)
'one is included in the notice' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is included in' indicates putting letters inside.
'the notice' becomes 'ad' (both can mean an advertisement).
'i' going within 'ad' is 'AID'.
(Other definitions for aid that I've seen before include "Care and help" , "Care, assistance" , "assistant" , "Relief" , "Help, assistance" .)