Reasonably OK, as a slow car may be? (8)
I believe the answer is:
I believe this is a double definition.
'reasonably ok' is the first definition.
(passable can mean of acceptable quality)
'as a slow car may be?' is the second definition.
(a slow car can be passed)
(Other definitions for passable that I've seen before include "Fair" , "examinations should be!" , "okay?" , "not hogging the road" , "Fairly good but not outstanding" .)