Gallagher's intro in dreadful song (5)
I believe the answer is:
dirge
'song' is the definition.
(I know that dirge is a type of song)
'gallagher's intro in dreadful' is the wordplay.
'intro' suggests taking the first letters.
'in' indicates putting letters inside.
'dreadful' becomes 'dire' (synonyms).
The first letter of 'gallagher' is 'g'.
'g' going within 'dire' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "Funeral hymn" , "feature of funeral" , "Mournful chant" , "Funeral lament" , "Threnody" .)