An obsession or an aim that's crazy? (5)
I believe the answer is:
mania
'an obsession' is the definition.
(I've seen this before)
'an aim that's crazy?' is the wordplay.
'that's crazy?' indicates an anagram (I've seen 'crazy' mean this).
'an'+'aim'='anaim'
'anaim' is an anagram of 'MANIA'.
'or' is the link.
(Other definitions for mania that I've seen before include "Madness - craze" , "Mental disorder that results in excessive or violent responses" , "Obsessional enthusiasm" , "fixation" , "serious obsession" .)