Aide altered impression (4)
I believe the answer is:
idea
'impression' is the definition.
(thesaurus)
'aide altered' is the wordplay.
'altered' indicates anagramming the letters.
'aide' with letters rearranged gives 'IDEA'.
(Other definitions for idea that I've seen before include "Notion in the mind" , "Plato's Form is" , "Thought, conception" , "Theory" , "Proposal" .)