Speed is 1,000 and 100 in reverse (4)
I believe the answer is:
knot
'speed' is the definition.
(knot is a nautical measure of speed)
'1000 and 100 in reverse' is the wordplay.
'1000' becomes 'k' (synonyms).
'and' is a charade indicator (letters next to each other).
'100' becomes 'ton' (ton can informally mean a hundred).
'in reverse' is a reversal indicator.
'ton' written backwards gives 'not'.
'k'+'not'='KNOT'
'is' acts as a link.
(Other definitions for knot that I've seen before include "Ravel" , "Measure of speed at sea" , "Tie (or what is put in one?)" , "It may be tied" , "measure of pace" .)