Turned up noise to conceal Military Intelligence - special release (8)
I believe the answer is:
emission
'release' is the definition.
(emission is a kind of release)
'turned up noise to conceal military intelligence special' is the wordplay.
'turned up' says the letters should be written backwards.
'to conceal' means one lot of letters goes inside another.
'military intelligence' becomes 'mi' (as in MI5 or MI6 - British military intelligence).
'special' becomes 's' (common abbreviation as in Special Air Service).
'noise' reversed gives 'esion'.
'mi'+'s'='mis'
'esion' going around 'mis' is 'EMISSION'.
(Other definitions for emission that I've seen before include "issue" , "A giving out or putting forth" , "Something given out" , "Going out" , "Is in some (anag) - discharge" .)