Quickly write section on swimming stroke (6)
I believe the answer is:
scrawl
'quickly write' is the definition.
(to scrawl is to write quickly)
'section on swimming stroke' is the wordplay.
'section' becomes 's' (legal abbreviation).
'on' is a charade indicator (letters next to each other).
'swimming stroke' becomes 'crawl' (front crawl).
's'+'crawl'='SCRAWL'
(Other definitions for scrawl that I've seen before include "Untidy writing" , "Illegible hand" , "Write carelessly or badly" , "Scribbled writing" , "write hurriedly" .)