Grown-up desperate to secure extremely destructive explosive (9)
I believe the answer is:
gunpowder
'explosive' is the definition.
(gunpowder is a kind of explosive)
'grown-up desperate to secure extremely destructive' is the wordplay.
'desperate' indicates an anagram (a desperate or pathetic spelling).
'to secure' is an insertion indicator (inserted letters are secured inside).
'extremely' means to remove the middle letters (letters on the extremes of the word are used).
'destructive' with its middle removed is 'de'.
'grown'+'up'='grownup'
'grownup' anagrammed gives 'gunpowr'.
'gunpowr' placed around 'de' is 'GUNPOWDER'.
(Other definitions for gunpowder that I've seen before include "and 24: 1605 conspiracy" , "Explosive invented in 9th-century China" , "Oldest known explosive" , "Explosive mixture (of tea?)" , "plot? Sort of" .)