Spirit in host, pure sadness (10)
I believe the answer is:
melancholy
'sadness' is the definition.
(I know that melancholy is a type of sadness)
'spirit in host pure' is the wordplay.
'spirit' becomes 'elan' (elan is a kind of spirit).
'in' indicates putting letters inside.
'host' becomes 'mc' (short for Master of Ceremonies).
'pure' becomes 'holy'.
'elan' going inside 'mc' is 'melanc'.
'melanc'+'holy'='MELANCHOLY'
(Other definitions for melancholy that I've seen before include "Gloomy, sad" , "sombre" , "Low spirits" , "Long-lasting sadness" , "Sad - tendency to depression" .)