Whisky smuggled into school with unknown sloppiness (8)
I believe the answer is:
schmaltz
'sloppiness' is the definition.
'whisky smuggled into school with unknown' is the wordplay.
'whisky' becomes 'malt' (malt whisky is a kind of whisky**).
'smuggled into' means one lot of letters goes inside another.
'school' becomes 'sch'.
'with' means one lot of letters go next to another.
'unknown' becomes 'z' (algebra).
'sch'+'z'='schz'
'malt' going within 'schz' is 'SCHMALTZ'.
(Other definitions for schmaltz that I've seen before include "Sickly sentimentality" , "Extreme or excessive sentimentality" , "Excessive sentimentality in art or music" , "Mushiness" , "mawkishness" .)