World title cracked by amateur (4)
I believe the answer is:
mars
'world' is the definition.
(I've seen this before)
'title cracked by amateur' is the wordplay.
'title' becomes 'mrs' (I have seen 'Female title' mean 'mrs' so perhaps 'title' could also mean 'mrs'**).
'cracked by' means one lot of letters goes inside another (to crack can mean to create a divide in).
'amateur' becomes 'a'.
'mrs' enclosing 'a' is 'MARS'.
(Other definitions for mars that I've seen before include "Injures - god of war" , "Famous Texas mission" , "one associated with war" , "Next planet out" , "Red planet and god of war" .)