Merry band's leader ignited high explosive (6)
I believe the answer is:
blithe
'merry' is the definition.
(I've seen this before)
'band's leader ignited high explosive' is the wordplay.
'leader' indicates taking the first letters.
'ignited' becomes 'lit' (synonyms).
'high explosive' becomes 'he'.
The first letter of 'bands' is 'b'.
'b'+'lit'+'he'='BLITHE'
(Other definitions for blithe that I've seen before include "Happy and maybe casual" , "Innocently happy" , "Gay" , "Uncaring" , "Cheerfully indifferent" .)