Mysterious secretion giving off nitrogen is unstable (8)
I believe the answer is:
esoteric
'mysterious' is the definition.
(I've seen this before)
'secretion giving off nitrogen is unstable' is the wordplay.
'giving off' suggests deleting specific letters.
'nitrogen' becomes 'n' ('N' can be a synonym of 'nitrogen').
'is unstable' indicates an anagram.
'secretion' with 'n' taken out is 'secretio'.
'secretio' with letters rearranged gives 'ESOTERIC'.
(Other definitions for esoteric that I've seen before include "Arcane -- coteries (anag)" , "just seen by specialist" , "Understandable by only an enlightened inner circle" , "Privileged or confidential among coteries" , "Taught to a select few" .)