He’d turned up late, turning up with vehicle finally dry (9)
I believe the answer is:
'dry' is the definition.
(dehydrating is a kind of drying)
'he'd turned up late turning up with vehicle finally' is the wordplay.
'turned up' shows that the letters should be reversed in order.
'late' becomes 'tardy' ('tardy' can be a synonym of 'late'**).
'turning up' says the letters should be written in reverse.
'with' says to put letters next to each other.
'finally' says to take the final letters.
The final letter of 'vehicle' is 'e'.
'hed' backwards is 'deh'.
'tardy' in reverse letter order is 'ydrat'.
(Other definitions for dehydrate that I've seen before include "Remove moisture content from, dry out" , "Preserve by removing all liquid from" , "Remove water from" , "Drain" , "Remove moisture from, make dry" .)