Alluding to umpire making a mistake (9)
I believe the answer is:
referring
'alluding' is the definition.
(I've seen this before)
'umpire making a mistake' is the wordplay.
'umpire' becomes 'ref' (short for referee).
'making a mistake' becomes 'erring' ('err' can be a synonym of 'make a mistake').
'ref'+'erring'='REFERRING'
'to' is the link.
(Other definitions for referring that I've seen before include "Consulting" , "Alluding, bringing up" , "Directing for information or having relation to" , "Adverting" , "Pertaining, relating" .)