Amount in Student Union: million (3)
I believe the answer is:
sum
'amount' is the definition.
(I know that amount can be written as sum)
'student union million' is the wordplay.
'student' becomes 's' (this could be a standard abbreviation which I don't know about).
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'million' becomes 'm' (abbreviation).
's'+'u'+'m'='SUM'
'in' is the link.
Can you help me to learn more?
(Other definitions for sum that I've seen before include "Add together" , "Arithmetic calculation" , "Total; problem" , "Amount of money; total" , "Total, add" .)