Sullen doggers regularly scrubbed vehicles (7)
I believe the answer is:
sledges
'vehicles' is the definition.
(sledge is a kind of vehicle)
'sullen doggers regularly scrubbed' is the wordplay.
'regularly scrubbed' indicates alternate letters (letters removed at regular intervals).
'sullen'+'doggers'='sullendoggers'
The alternating letters of 'sullendoggers' are 'SLEDGES'.
(Other definitions for sledges that I've seen before include "Sleighs; hammers" , "Snow carts" , "insults" , "More than one big hitter" .)