Almost certain aspect will be lacking depth (7)
I believe the answer is:
surface
'lacking depth' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'almost certain aspect' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'certain' becomes 'sure' (synonyms).
'aspect' becomes 'face' (side of a building).
'sure' with its final letter taken away is 'sur'.
'sur'+'face'='SURFACE'
'will be' is the link.
Can you help me to learn more?
(Other definitions for surface that I've seen before include "skin" , "Facade" , "Outer appearance" , "Top spot" , "Upper limit of liquid" .)