Irrational to criticise thief (6)
I believe the answer is:
pirate
'thief' is the definition.
(I know that pirate is a type of thief)
'irrational to criticise' is the wordplay.
'irrational' becomes 'pi' (an irrational number).
'to criticise' becomes 'rate'.
'pi'+'rate'='PIRATE'
(Other definitions for pirate that I've seen before include "Illegally-copied (version)" , "Cornishman maybe" , "Sea-robber" , "Sea raider" , "Penzance marauder ?" .)