Million in shillings is mine? (4)
I believe the answer is:
bomb
'is mine?' is the definition.
(I've seen this before)
'million in shillings' is the wordplay.
'million' becomes 'm' (abbreviation).
'in' means one lot of letters goes inside another.
'shillings' becomes 'bob' (plural as in 'two bob').
'm' put inside 'bob' is 'BOMB'.
(Other definitions for bomb that I've seen before include "Flop - load of money" , "Mine" , "Great deal of money (slang)" , "Failure" , "Drop" .)