Held in the button -- turned over TV (4)
I believe the answer is:
tube
'tv' is the definition.
(I've seen this in another clue)
'held in the button turned over' is the wordplay.
'held in' indicates the answer is hidden within the clue.
'turned over' shows that the letters should be reversed in order.
'ebut' is hidden within 'the button'.
'ebut' in reverse letter order is 'TUBE'.
(Other definitions for tube that I've seen before include "Hollow cylindrical object" , "Can of beer (colloquial)" , "Long, hollow cylinder" , "Cylindrical hollow conduit" , "Pipe; underground train" .)