Group's leader cutting dreadful lament (5)
I believe the answer is:
dirge
'lament' is the definition.
('dirge' can be a synonym of 'lament')
'group's leader cutting dreadful' is the wordplay.
'leader' indicates taking the first letters.
'cutting' indicates putting letters inside (some letters cut their way into a word).
'dreadful' becomes 'dire' ('dire' can be a synonym of 'dreadful').
The initial letter of 'group' is 'g'.
'g' placed into 'dire' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "part of funeral, perhaps" , "Slow, sad song" , "Keen; lament" , "Dragging piece" , "Sombre music" .)