Muted trombone's finale, playing hymn (2,4)
I believe the answer is:
te deum
'hymn' is the definition.
(Te Deum is an example)
'muted trombone's finale playing' is the wordplay.
'finale' suggests the final letters.
'playing' indicates an anagram.
The last letter of 'trombone' is 'e'.
'muted'+'e'='mutede'
'mutede' is an anagram of 'TE DEUM'.
(Other definitions for te deum that I've seen before include "'Thee, O God' (Lat.)" , "Ancient Latin hymn" , "Canticle" , "Early Christian hymn of praise to God (Latin)" , "Christian hymn of thanksgiving" .)