Ultimately careless stroke -- that's hard to read (6)
I believe the answer is:
scrawl
'hard to read' is the definition.
I can't tell whether this definition defines the answer.
'ultimately careless stroke' is the wordplay.
'ultimately careless' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'stroke' becomes 'crawl' (crawl is a kind of stroke).
's'+'crawl'='SCRAWL'
'that's' is the link.
Can you help me to learn more?
(Other definitions for scrawl that I've seen before include "Write illegibly" , "Hurried writing" , "Write carelessly or sloppily" , "London's river" , "Write hastily" .)