Son runs off and goes fast (7)
I believe the answer is:
sprints
'goes fast' is the definition.
'son runs off' is the wordplay.
'son' becomes 's'.
'runs off' becomes 'prints'.
's'+'prints'='SPRINTS'
'and' is the link.
(Other definitions for sprints that I've seen before include "careers" , "Darts" , "Dashes" .)