Probe deep mental impression (5)
I believe the answer is:
sound
This clue is a quadruple definition!!
'probe' is the first definition.
(thesaurus)
'deep' is the second definition.
(synonyms)
'mental' is the third definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'impression' is the fourth definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for sound that I've seen before include "Healthy; narrow channel" , "Health - noise" , "note, for instance" , "stretch of water" , "Safe" .)