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 there's an association between them I don't understand?
'one of them catching the dangling rein' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'the' (both can introduce a definite example of something) and 'the' is present in the answer.
an anagram of 'rein' is 'neir' which is within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
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" .)