In no case loose the rein (7)
I believe the answer is:
neither
'in no case' is the definition.
'loose the rein' is the wordplay.
'loose' indicates anagramming the letters (letters are loosened from their original positions).
'the'+'rein'='therein'
'therein' is an anagram of 'NEITHER'.
(Other definitions for neither that I've seen before include "None of the two" , "In three (anag.)" , "Negative word" , "And not; nor yet" , "double negative?" .)