Witness established uncertainty in row (9)
I believe the answer is:
testifier
'witness' is the definition.
(testifier is a kind of witness)
'established uncertainty in row' is the wordplay.
'established' becomes 'est' (I've seen this in other clues).
'uncertainty' becomes 'if' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'row' becomes 'tier' (tier is a kind of row**).
'est'+'if'='estif'
'estif' inserted within 'tier' is 'TESTIFIER'.
(Other definitions for testifier that I've seen before include "One who may declare" , "Witness" , "person in court?" , "One acknowledging" .)