Light wood used by Hannibal's army? (5)
I believe the answer is:
balsa
'light wood' is the definition.
(I've seen this before)
'used by hannibal's army?' is the wordplay.
'used by' says the answer is hidden in the clue.
'BALSA' is hidden in the letters of 'hannibals army'.
(Other definitions for balsa that I've seen before include "Strong, light wood" , "Something light" , "lightweight boat-builder" , "South American tree" , "Central American forest tree used to make floats, rafts, models etc" .)