Our arms once held this grown-up struggling to accept the ends of childhood innocence (9)
I believe the answer is:
gunpowder
'our arms once held this' is the definition.
The definition suggests a singular noun which matches the answer.
'grown-up struggling to accept the ends of childhood innocence' is the wordplay.
'struggling' is an anagram indicator (the letters struggle or wriggle into a new form).
'to accept' is an insertion indicator.
'the ends of' suggests the final letters (I've seen 'ending of' mean this).
The final letters of 'childhood innocence' are 'de'.
'grown'+'up'='grownup'
'grownup' anagrammed gives 'gunpowr'.
'gunpowr' enclosing 'de' is 'GUNPOWDER'.
Can you help me to learn more?
(Other definitions for gunpowder that I've seen before include "Explosive mixture (of tea?)" , "Explosive invented in 9th-century China" , "Explosive chemical mixture" , "Explosive mixture - plot of 1605" , "It's used in gunnery and fireworks" .)