Left expert holding knight's weapon (5)
I believe the answer is:
lance
'weapon' is the definition.
(lance is a kind of weapon)
'left expert holding knight's' is the wordplay.
'left' becomes 'l' (common abbreviation).
'expert' becomes 'ace' (ace is a kind of expert).
'holding' indicates putting letters inside.
'knight' becomes 'n' (chess).
'ace' placed around 'n' is 'ance'.
'l'+'ance'='LANCE'
(Other definitions for lance that I've seen before include "Clean (anag) -- weapon" , "Long pointed weapon of old" , "Incise" , "A horse soldier's spear" , "One like pike" .)