Prevailing mood at Missouri field (10)
I believe the answer is:
atmosphere
'prevailing mood' is the definition.
(atmosphere can mean a mood or air)
'at missouri field' is the wordplay.
'missouri' becomes 'mo' (abbreviation for the US state).
'field' becomes 'sphere' (synonyms).
'at'+'mo'+'sphere'='ATMOSPHERE'
(Other definitions for atmosphere that I've seen before include "Spirit" , "We are always enveloped in this" , "blue?" , "Gas surrounding a planet" , "Surrounding ambience" .)