Odd circus characters strike at the heart (4)
I believe the answer is:
crux
'the heart' is the definition.
(I've seen this in another clue)
'odd circus characters strike' is the wordplay.
'odd' means one should take alternating letters.
'characters strike' becomes 'x' (I can't justify this - if you can you should give a lot more credence to this answer).
The alternating letters of 'circus' are 'cru'.
'cru'+'x'='CRUX'
'at' acts as a link.
Can you help me to learn more?
(Other definitions for crux that I've seen before include "Particular difficulty" , "Main point (of the matter)" , "Most important point at issue" , "Key point" , "Core" .)