Distance equal to time (6)
I believe the answer is:
parsec
'distance' is the definition.
(I've seen this before)
'equal to time' is the wordplay.
'equal' becomes 'par'.
'to time' becomes 'sec' (**).
'par'+'sec'='PARSEC'
(Other definitions for parsec that I've seen before include "3.26 light years" , "unit for measuring distances" , "Measure of distance in space; spacer (anag.)" , "Large unit of distance" , "Three-and-a-bit light-years" .)