Contrary forces rapidly covering a huge distance (6)
I believe the answer is:
parsec
'a huge distance' is the definition.
'contrary forces rapidly covering' is the wordplay.
'contrary' shows that the letters should be reversed in order.
'covering' indicates a hidden word.
'cesrap' is hidden within 'forces rapidly'.
'cesrap' reversed gives 'PARSEC'.
(Other definitions for parsec that I've seen before include "fair distance" , "three and a quarter twenty sevens [LIGHT YEARS]" , "it takes years to get this far" , "Three-and-a-bit light-years" , "3.26 light years" .)