Obscure heart of underworld border causes spasm for a learner (10)
I believe the answer is:
elliptical
'obscure heart of underworld border' is the definition.
'elliptical' can be an answer for 'obscure' (I've seen this before). I am not certain of the 'heart of underworld border' bit.
'spasm for a learner' is the wordplay.
'spasm' becomes 'elliptic' (I can't justify this - if you can you should give a lot more credence to this answer).
'for' says to put letters next to each other.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'elliptic'+'a'+'l'='ELLIPTICAL'
'causes' is the link.
Can you help me to learn more?
(Other definitions for elliptical that I've seen before include "regarding the motion of planets" , "Like words omitted" , "Obscure" , "description of nightmarish billiard balls?" , "Oval" .)