Muscles stretch -- it's a kind of art (8)
I believe the answer is:
abstract
'a kind of art' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'muscles stretch' is the wordplay.
'muscles' becomes 'abs' (short for abdominal muscles).
'stretch' becomes 'tract' (a tract is a stretch of land).
'abs'+'tract'='ABSTRACT'
'it's' is the link.
Can you help me to learn more?
(Other definitions for abstract that I've seen before include "Academic" , "Half-inch" , "Remove; theoretical" , "Take away - immaterial" , "piece of artwork" .)