Laugh at round ends? It's tough (4)
I believe the answer is:
Here is my best explanation:
'tough' is the definition.
('hard' is associated in meaning with 'tough')
'laugh at round ends?' is the wordplay.
'laugh' becomes 'ha'.
'at' means one lot of letters go next to another.
'ends?' says to hollow out the word (remove centre letters) (I've seen 'ends' mean this).
'round' with its middle taken out 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".)