Weapon kept by clan certainly (5)
I believe the answer is:
lance
'weapon' is the definition.
(lance is a kind of weapon)
'kept by clan certainly' is the wordplay.
'kept by' indicates the answer is hidden within the clue.
'LANCE' is hidden within 'clan certainly'.
(Other definitions for lance that I've seen before include "Incise" , "Boy - prick (a boil)" , "Jousting weapon" , "Horseman's long weapon" , "steel" .)