Rare problem with irrational weapon (6)
I believe the answer is:
rapier
'weapon' is the definition.
(rapier is a kind of weapon)
'rare problem with irrational' is the wordplay.
'problem' indicates anagramming the letters.
'with' is an insertion indicator.
'irrational' becomes 'pi' (an irrational number).
'rare' with letters rearranged gives 'raer'.
'raer' placed around 'pi' is 'RAPIER'.
(Other definitions for rapier that I've seen before include "Thrusting weapon" , "A fencing sword for repair" , "fencing equipment" , "Sword for thrusting" , "Straight narrow sword with two edges" .)