Move across to pack explosive in missile system (7)
I believe the answer is:
trident
'missile system' is the definition.
(UK missile system)
'move across to pack explosive' is the wordplay.
'move across' becomes 'ride' (riding is a type of moving).
'to pack' is an insertion indicator.
'explosive' becomes 'TNT' (TNT is an example).
'ride' going within 'tnt' is 'TRIDENT'.
'in' is the link.
(Other definitions for trident that I've seen before include "3-pronged spear" , "Fork perhaps" , "spear carried at sea" , "Three-pointed spear" , "Nuclear missile" .)