Tesla's running flat (5)
I believe the answer is:
stale
'flat' is the definition.
(I've seen this in another clue)
'tesla's running' is the wordplay.
'running' is an anagram indicator (the letters run around to different places).
'tesla' with letters rearranged gives 'STALE'.
(Other definitions for stale that I've seen before include "Musty or hackneyed" , "Not fresh, being least" , "no longer bearing interest?" , "Least (anag.)" , "No longer fresh" .)