Nothing appears to add up in this thin material (6)
I believe the answer is:
muslin
'material' is the definition.
(I know that muslin is a type of material)
'nothing appears to add up in this thin' is the wordplay.
I cannot really understand how this works, but
'in' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for muslin that I've seen before include "Fine soft plain-woven cotton fabric of gauzy appearance" , "dress material" , "Gauze-like cloth" , "Cotton for veil or strainer" , "Light cotton cloth" .)