One stops dog seizing line (4)
I believe the answer is:
plug
'one stops' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'dog seizing line' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog).
'seizing' is an insertion indicator (inserted letters are seized).
'line' becomes 'l' (used when specifying particular lines from a poem).
'pug' enclosing 'l' is 'PLUG'.
Can you help me to learn more?
(Other definitions for plug that I've seen before include "Blatant promotion" , "Piece of tobacco; block" , "Power point" , "Mention favourably - stopper" , "Stuff" .)