Small toboggan circling start of deep mud (6)
I believe the answer is:
sludge
'mud' is the definition.
(I've seen this before)
'small toboggan circling start of deep' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'toboggan' becomes 'luge' (synonyms).
'circling' indicates putting letters inside (circling can mean going around or containing).
'start of' indicates taking the first letters.
The initial letter of 'deep' is 'd'.
'luge' enclosing 'd' is 'ludge'.
's'+'ludge'='SLUDGE'
(Other definitions for sludge that I've seen before include "Soft mud; slimy waste" , "Dirty oil; soft wet mud" , "Muddy sediment" , "Ooze" , "Thick greasy mud" .)