Not one of them catching the dangling rein (7)
I believe the answer is:
neither
'not' is the definition.
Both the definition and answer are adverbs. Perhaps they are linked in a way I don't understand?
'one of them catching the dangling rein' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
an anagram of 'the' is 'eth' which is within the remaining letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for neither that I've seen before include "Not one thing or the other" , "Therein (anag)" , "double negative?" , "The rein (anag.)" , "both are rejected" .)