Number Ten initially backtracking somewhat (4)
I believe the answer is:
nine
'number' is the definition.
(nine is a number)
'ten initially backtracking somewhat' is the wordplay.
'somewhat' indicates a hidden word.
'backtracking' shows that the letters should be reversed in order.
'enin' can be found hidden inside 'ten initially'.
'enin' backwards is 'NINE'.
(Other definitions for nine that I've seen before include "A perfect square" , "Square next up after four" , "Square root of 81" , "Number of lepers who failed to return" , "Short time for a wonder" .)