Amount gained tenner, I'm excited about cash initially (9)
I believe the answer is:
increment
'amount gained' is the definition.
(as in incremental gains)
'tenner i'm excited about cash initially' is the wordplay.
'excited' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'initially' indicates taking the first letters.
The first letter of 'cash' is 'c'.
'tenner'+'im'='tennerim'
'tennerim' with letters rearranged gives 'inrement'.
'inrement' placed around 'c' is 'INCREMENT'.
(Other definitions for increment that I've seen before include "Rise" , "Addition to wages etc" , "Step" , "Increase (one of a series)" , "growth" .)