Laugh at round ends? It's tough (4)
I believe the answer is:
Here is my best explanation:
'tough' is the definition.
('hard' can be a synonym of 'tough')
'laugh at round ends?' is the subsidiary indication.
'laugh' becomes 'ha'.
'at' says to put letters next to each other.
'ends?' suggests removing the centre.
'round' with its centre removed is 'rd'.
'it's' is the link.
(Other definitions for hard that I've seen before include "Novel featuring Mr Gradgrind", "badly", "Strenuous", "Rigid", "Intensely", "Unyielding", "impenetrable", "Difficult - firm", "to be listened to in Scotland" and "Difficult; rigid".)