Motorway divides Lough in Royal Trim (6)
I believe the answer is:
ermine
'royal trim' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'motorway divides lough' is the wordplay.
'motorway' becomes 'mi' (resembles M1, English motorway).
'divides' is an insertion indicator.
'lough' becomes 'erne'.
'mi' placed inside 'erne' is 'ERMINE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for ermine that I've seen before include "In mere strange fur" , "Winter fur of the stoat" , "Stoat in northern regions" , "Animal with brown fur that turns white in winter" , "Stoat in its winter coat" .)