Stay with boy to include day in France (7)
I believe the answer is:
sojourn
'stay' is the definition.
(sojourn is a kind of stay)
'boy to include day in france' is the wordplay.
'boy' becomes 'son' (synonyms).
'to include' is an insertion indicator.
'day in france' becomes 'jour' (I can't justify this - if you can you should give a lot more credence to this answer).
'son' going around 'jour' is 'SOJOURN'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for sojourn that I've seen before include "Relaxed, tolerant" , "Stopover" , "Short stay" , "Stop" , "Temporary residence" .)