Right up inside smooth thread (6)
I believe the answer is:
strand
'thread' is the definition.
(thesaurus)
'right up inside smooth' is the wordplay.
'right' becomes 'rt'.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'inside' is an insertion indicator.
'smooth' becomes 'sand' (sanding is a kind of smoothing**).
'rt' in reverse letter order is 'tr'.
'tr' put into 'sand' is 'STRAND'.
(Other definitions for strand that I've seen before include "Beach or shore" , "Shore; single thread" , "Stretch of sand" , "Leave high and dry" , "Maroon - bit of hair" .)