Oddly glum and quiet outpouring (4)
I believe the answer is:
gush
'outpouring' is the definition.
(thesaurus)
'oddly glum and quiet' is the wordplay.
'oddly' indicates alternate letters.
'and' says to put letters next to each other.
'quiet' becomes 'sh'.
The alternate letters of 'glum' are 'gu'.
'gu'+'sh'='GUSH'
(Other definitions for gush that I've seen before include "Speak effusively" , "Speak/write effusively" , "Pour forth" , "Spurt" , "Pour out - appear overly effusive" .)