Write, almost illegibly, "sun-stroke" (6)
I believe the answer is:
scrawl
'write' is the definition.
(scrawling is a kind of writing)
'sun-stroke' is the wordplay.
'sun' becomes 'S' (abbreviation).
'stroke' becomes 'crawl' (crawl is a kind of stroke).
's'+'crawl'='SCRAWL'
'almost illegibly' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for scrawl that I've seen before include "Quickly pen" , "Write untidily" , "Scribble" , "Hasty writing" , "Write carelessly or sloppily" .)