Unwholesome air of missing serviceman's mother (6)
I believe the answer is:
miasma
'unwholesome air' is the definition.
(I've seen this before)
'missing serviceman's mother' is the wordplay.
'missing serviceman' becomes 'mias' (I can't justify this - if you can you should believe this answer much more).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'mias'+'ma'='MIASMA'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for miasma that I've seen before include "Unwholesome atmosphere, unhealthy vapors" , "In poor atmospheric conditions" , "Foul vapour" , "Oppressive atmosphere" , "Foul vapours or unwholesome air" .)