Requests one to come and see amount used on the first day by son (7)
I believe the answer is:
summons
'requests one to come and' is the definition.
I can't tell whether this definition defines the answer.
'amount used on the first day by son' is the wordplay.
'amount' becomes 'sum' (synonyms).
'used on' means one lot of letters go next to another.
'the first day' becomes 'mon' (short for Monday. I am not sure about the 'first' bit.).
'by' says to put letters next to each other.
'son' becomes 's' (genealogical abbreviation for son).
'sum'+'mon'+'s'='SUMMONS'
'see' acts as a link.
Can you help me to learn more?
(Other definitions for summons that I've seen before include "Order to attend court" , "orders" , "Calls to appear" , "Order to appear in law court" , "Call to join" .)