Plan certain to reveal spear (5)
I believe the answer is:
lance
'spear' is the definition.
(I know this)
'plan certain to reveal' is the wordplay.
'to reveal' indicates a hidden word.
'LANCE' can be found hidden inside 'plan certain'.
(Other definitions for lance that I've seen before include "Shafted weapon used on horseback" , "A horse soldier's spear" , "Long, pointed weapon (5)." , "fish" , "Surgical knife" .)