Dash off, seeing son move slowly (6)
I believe the answer is:
scrawl
'dash off' is the definition.
(dash off can mean to write quickly)
'son move slowly' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'move slowly' becomes 'crawl' (I've seen this before).
's'+'crawl'='SCRAWL'
'seeing' is the link.
(Other definitions for scrawl that I've seen before include "Long, maybe illegible, piece of writing" , "Write clumsily, hardly legible" , "careless writing" , "write hurriedly" , "Crawls away from bad writing" .)