Almost certain aspect will be lacking depth (7)
I believe the answer is:
surface
'lacking depth' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe there's an association between them I don't understand?
'almost certain aspect' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'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" .)