Mystical system Henry introduced to eccentric Soho type (9)
I believe the answer is:
theosophy
'mystical system' is the definition.
(mystical religion)
'henry introduced to eccentric soho type' is the wordplay.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'introduced to' is an insertion indicator.
'eccentric' indicates an anagram.
'soho'+'type'='sohotype'
'sohotype' with letters rearranged gives 'theosopy'.
'h' placed within 'theosopy' is 'THEOSOPHY'.
(Other definitions for theosophy that I've seen before include "belief system" , "divine thoughts?" , "Hope to shy (anag) - mystical belief" , "thought about religion" , "religious system" .)