Delicate material covering top of nuclear weapon (5)
I believe the answer is:
lance
'weapon' is the definition.
(I know that lance is a type of weapon)
'delicate material covering top of nuclear' is the wordplay.
'delicate material' becomes 'lace' (I've seen this before).
'covering' indicates putting letters inside.
'top of' indicates taking the first letters.
The first letter of 'nuclear' is 'n'.
'lace' enclosing 'n' is 'LANCE'.
(Other definitions for lance that I've seen before include "Old cavalry weapon" , "Pierce (a boil)" , "A horse soldier's spear" , "Spear carried by mounted soldier" , "Cut open" .)