(√-1 + 50) ÷ 50? That's difficult (3)
I believe the answer is:
ill
'difficult' is the definition.
(ill could historically mean difficult)
'1 50 50?' is the wordplay.
'i' becomes 'i' (Roman numeral).
'50' becomes 'l' (Roman numeral).
'50?' becomes 'l' (Roman numeral).
'i'+'l'+'l'='ILL'
'that's' is the link.
(Other definitions for ill that I've seen before include "Unwell, sick" , "Afflicted" , "Wicked" , "Harmful" , "Imperfectly; unwell" .)