Practise on undertaker’s vehicle (8)
I believe the answer is:
rehearse
'practise' is the definition.
(I know that practise can be written as rehearse)
'on undertaker's vehicle' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'undertaker's vehicle' becomes 'hearse' (I've seen this before).
're'+'hearse'='REHEARSE'
(Other definitions for rehearse that I've seen before include "Practise parts" , "Run through (a play)" , "Practise like actor" , "Practise before performing in public" , "Cast practise for play" .)