A girl lies about weapons (8)
I believe the answer is:
missiles
'weapons' is the definition.
(missile is a kind of weapon)
'a girl lies about' is the wordplay.
'a girl' becomes 'miss' (synonyms).
'about' indicates an anagram.
'lies' anagrammed gives 'iles'.
'miss'+'iles'='MISSILES'
(Other definitions for missiles that I've seen before include "Projectiles" , "Things thrown or shells fired" , "Projected weapons" .)