State umpire backs a cracker (5)
I believe the answer is:
wafer
'a cracker' is the definition.
(I've seen this in another clue)
'state umpire backs' is the wordplay.
'state' becomes 'wa' (I've seen this in other clues).
'umpire' becomes 'ref' (short for referee).
'backs' is a reversal indicator.
'ref' back-to-front is 'fer'.
'wa'+'fer'='WAFER'
(Other definitions for wafer that I've seen before include "Thin type" , "Host's" , "Very thin 14 Down" , "Very thin slice" , "Ice-cream biscuit" .)