Medieval study of lacy hem (7)
I believe the answer is:
alchemy
'medieval study' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'of lacy hem' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'lacy'+'hem'='lacyhem'
'lacyhem' anagrammed gives 'ALCHEMY'.
Can you help me to learn more?
(Other definitions for alchemy that I've seen before include "process of going for gold" , "Chemistry's forerunner" , "Pre-scientific chemistry" , "Medieval forerunner of chemistry" , "Old pseudoscience hoping to turn base metals into gold" .)