Manuscript about the mysterious spirits (5)
I believe the answer is:
meths
'spirits' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.
'manuscript about the mysterious' is the wordplay.
'manuscript' becomes 'ms' (abbreviation for manuscript).
'about' is an insertion indicator.
'mysterious' is an anagram indicator.
'the' is an anagram of 'eth'.
'ms' placed around 'eth' is 'METHS'.
Can you help me to learn more?
(Other definitions for meths that I've seen before include "unsuitable alcohol" , "Spirit" , "Violet-coloured alcohol" , "Fuel" , "Methylated spirits (colloq.)" .)