Son's first bed delivered without charge (4-4)
I believe the answer is:
scot-free
'without charge' is the definition.
(to get off scot-free is to avoid punishment)
'son's first bed delivered' is the wordplay.
'son's first' becomes 's' (fst letter of 'son').
'bed' becomes 'cot' (cot is a type of bed).
'delivered' becomes 'free' (I can't explain this - if you can you should believe this answer much more).
's'+'cot'+'free'='SCOT-FREE'
Can you help me to learn more?
(Other definitions for scot-free that I've seen before include "Without loss or penalty" , "Secret of being unpunished - like Jock?" , "Entirely without penalty or injury" , "See 12" , "Unscathed" .)