Fact is, Tesla breaks still (9)
I believe the answer is:
statistic
'fact' is the definition.
(I've seen this before)
'is tesla breaks still' is the wordplay.
'tesla' becomes 'T' (scientific unit).
'breaks' indicates putting letters inside (inserted letters break into the word).
'still' becomes 'static' (static can mean still or not moving).
'is'+'t'='ist'
'ist' inserted within 'static' is 'STATISTIC'.
(Other definitions for statistic that I've seen before include "part of a table perhaps" , "One single item of data" , "Data point" , "Piece of data" , "E.g. crime figure" .)