Aide's strange impression (4)
I believe the answer is:
idea
'impression' is the definition.
(I've seen this before)
'aide's strange' is the wordplay.
'strange' indicates an anagram.
'aide' with letters rearranged gives 'IDEA'.
(Other definitions for idea that I've seen before include "Notion in the mind" , "Intention" , "Theory" , "Plan" , "Product of the mind" .)