Reveal error after umpire's call (3,4)
I believe the answer is:
let slip
'reveal' is the definition.
(thesaurus)
'error after umpire's call' is the wordplay.
'error' becomes 'slip' (slip is a kind of error).
'after' says to put letters next to each other.
'umpire's call' becomes 'let' (I've seen this in another clue).
'slip' after 'let' is 'LET SLIP'.
(Other definitions for let slip that I've seen before include "Fail to keep Secret" , "accidentally revealed" , "Inadvertently reveal (something)" , "Allow to fall or reveal a secret accidentally" , "Reveal accidentally, or allow to skid?" .)