Mysterious European, very tense man (8)
I believe the answer is:
esoteric
'mysterious' is the definition.
(I've seen this before)
'european very tense man' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'very' becomes 'so' (I've seen this in other clues).
'tense' becomes 't' (abbreviation used in many dictionaries).
'man' becomes 'eric' (I've seen this before**).
'e'+'so'+'t'+'eric'='ESOTERIC'
(Other definitions for esoteric that I've seen before include "Taught to a select few" , "Hard to understand" , "Intelligible only to the initiated" , "Aimed at a few" , "Secret kind of coteries" .)