Coin tossed up in the middle? (4)
I believe the answer is:
'coin' is the definition.
(dime is a kind of coin)
'tossed up in the middle?' is the wordplay.
'tossed up' shows that the letters should be reversed in order.
'in' indicates the answer is hidden within the clue.
'emid' is hidden in the letters of 'the middle'.
'emid' written backwards gives 'DIME'.
(Other definitions for dime that I've seen before include "Ten-cent piece" , "Ten cents piece" , "Something round and silvery" , "Ten-cent coin in America" , "US ten cent piece" .)