Mary Jane and son drove a bit of an old crock (8)
I believe the answer is:
potsherd
'a bit of an old crock' is the definition.
(broken piece of a pot or crock)
'mary jane and son drove' is the wordplay.
'mary jane' becomes 'pot' (synonyms).
'and' is a charade indicator (letters next to each other).
'son' becomes 's' (genealogical abbreviation for son).
'drove' becomes 'herd' (I've seen this in another clue).
'pot'+'s'+'herd'='POTSHERD'
(Other definitions for potsherd that I've seen before include "bit of china" , "Pottery fragment; the drops (anag.)" , "part of vessel found in digs" , "A broken piece of earthenware in archeology" , "Piece once fired" .)