Write “September first”; after that, make slow progress (6)
I believe the answer is:
scrawl
'write' is the definition.
(scrawling is a kind of writing)
'september first after that make slow progress' is the wordplay.
'first' suggests taking the first letters.
'after' says to put letters next to each other.
'that make slow progress' becomes 'crawl' (I've seen this before).
The first letter of 'september' is 's'.
's'+'crawl'='SCRAWL'
(Other definitions for scrawl that I've seen before include "Write illegibly" , "Write in slovenly manner" , "Poor handwriting" , "Hurried writing" , "Roughly put down" .)