Write quickly seeing son move like a baby (6)
I believe the answer is:
scrawl
'write quickly' is the definition.
'son move like a baby' is the wordplay.
'son' becomes 's'.
'move like a baby' becomes 'crawl'.
's'+'crawl'='SCRAWL'
'seeing' acts as a link.
(Other definitions for scrawl that I've seen before include "careless writing" , "Write hastily" , "carelessly written" , "Write untidily" , "Crawls away from bad writing" .)