Small toboggan contains bit of dirt and slime (6)
I believe the answer is:
sludge
'slime' is the definition.
(synonyms)
'small toboggan contains bit of dirt' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'toboggan' becomes 'luge' (synonyms).
'contains' is an insertion indicator.
'bit of' indicates taking the first letters.
The initial letter of 'dirt' is 'd'.
'luge' enclosing 'd' is 'ludge'.
's'+'ludge'='SLUDGE'
'and' is the link.
(Other definitions for sludge that I've seen before include "Gloop, gunk, muck" , "Soft mud; slimy waste" , "'Muck, slime (6)'" , "Mire - sewage" , "Melting snow" .)