Oddly glum in manner, even speechless (6-4)
I believe the answer is:
tongue-tied
'speechless' is the definition.
(thesaurus)
'oddly glum in manner even' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'in' means one lot of letters goes inside another.
'manner' becomes 'tone'.
'even' becomes 'tied' (synonyms).
The alternating letters of 'glum' are 'gu'.
'gu' placed inside 'tone' is 'tongue'.
'tongue'+'tied'='TONGUE-TIED'
(Other definitions for tongue-tied that I've seen before include "Unable to speak coherently" , "Speechless with shyness" , "(Out of shyness) barely able to speak" , "Speechless from embarrassment" , "Unable to say" .)