Squirm after son's bad handwriting (6)
I believe the answer is:
scrawl
'bad handwriting' is the definition.
(untidy handwriting)
'squirm after son's' is the wordplay.
'squirm' becomes 'crawl' (as in your skin squirming or crawling).
'after' says to put letters next to each other.
'crawl' after 's' is 'SCRAWL'.
(Other definitions for scrawl that I've seen before include "Write carelessly or sloppily" , "write hurriedly" , "Messy drawing, possibly" , "Write hastily" , "Untidy writing" .)