Brilliant soldier hacked into computer (5)
I believe the answer is:
magic
'brilliant' is the definition.
(thesaurus)
'soldier hacked into computer' is the wordplay.
'soldier' becomes 'gi' (government issue or general issue).
'hacked into' means one lot of letters goes inside another.
'computer' becomes 'mac' (I've seen this before).
'gi' going inside 'mac' is 'MAGIC'.
(Other definitions for magic that I've seen before include "Superb" , "Art that supposedly invokes supernatural powers" , "Supernatural power" , "Mysterious quality" , "Enchantment" .)