Prove uncle Vincent is not all there (6)
I believe the answer is:
evince
'prove' is the definition.
(thesaurus)
'uncle vincent is not all there' is the wordplay.
'is not all there' indicates the answer is hidden within the clue (only some of the letters are to be retained).
'EVINCE' can be found hidden inside 'uncle vincent'.
(Other definitions for evince that I've seen before include "Express,show" , "Indicate or display, say a feeling" , "Indicate (something)" , "Prove beyond doubt" , "Manifest, show" .)