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' means one lot of letters goes inside another (inserted letters are secured inside).
'extremely' suggests removing the centre (letters on the extremes of the word are used).
'destructive' with its middle taken out is 'de'.
'grown'+'up'='grownup'
'grownup' anagrammed gives 'gunpowr'.
'gunpowr' going 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" .)