Write badly with small stroke (6)
I believe the answer is:
scrawl
'write badly' is the definition.
(I've seen this before)
'small stroke' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'stroke' becomes 'crawl' (crawl is a kind of stroke).
's'+'crawl'='SCRAWL'
'with' acts as a link.
(Other definitions for scrawl that I've seen before include "Hasty writing" , "careless writing" , "Write poorly, bad handwriting" , "Write clumsily, hardly legible" , "write hurriedly" .)