Scene is mistakenly introducing 100 subjects (8)
I believe the answer is:
sciences
'subjects' is the definition.
(I know this)
'scene is mistakenly introducing 100' is the wordplay.
'mistakenly' indicates anagramming the letters.
'introducing' means one lot of letters goes inside another (some letters introduced or inserted into others).
'100' becomes 'C' (Roman numeral).
'scene'+'is'='sceneis'
'sceneis' anagrammed gives 'scienes'.
'scienes' enclosing 'c' is 'SCIENCES'.
(Other definitions for sciences that I've seen before include "Branches of knowledge requiring systematic study and methods" , "Branches of learning" , "Biology and physics, say" , "Physics and chemistry, say" , "Branches of knowledge and skilful techniques" .)