Mood created by a most unusual soft present (10)
I believe the answer is:
atmosphere
'mood' is the definition.
(atmosphere is a kind of mood)
'a most unusual soft present' is the wordplay.
'unusual' is an anagram indicator.
'soft' becomes 'p' (musical abbreviation for 'piano').
'present' becomes 'here' (both can mean in attendance).
'most' is an anagram of 'tmos'.
'a'+'tmos'+'p'+'here'='ATMOSPHERE'
'created by' is the link.
(Other definitions for atmosphere that I've seen before include "Feeling" , "Gas surrounding a planet" , "Feel" , "Air" , "Spirit" .)