Attach and finish with a number to be added (6)
I believe the answer is:
addend
'attach and finish with a' is the definition.
I can't tell whether this definition defines the answer.
'number to be added' is the wordplay.
'number' becomes 'add' (adding is a kind of numbering).
'to be' becomes 'end' (ending is a kind of being).
'added' means one lot of letters go next to another.
'add'+'end'='ADDEND'
Can you help me to learn more?
(Other definitions for addend that I've seen before include "A number put on" , "something to put on" , "An increasing number" , "increasing quantity" .)