Song serious involving gravity (5)
I believe the answer is:
dirge
'song' is the definition.
(I know that dirge is a type of song)
'serious involving gravity' is the wordplay.
'serious' becomes 'dire' (I've seen this in another clue).
'involving' indicates putting letters inside.
'gravity' becomes 'g'.
'dire' placed around 'g' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "lamentable composition" , "Funereal music" , "Funeral hymn" , "Funereal hymn" , "Song or hymn of mourning" .)