Many a red deer has fled trembling inside (4,4)
I believe the answer is:
hard left
'many a red deer' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'has fled trembling inside' is the wordplay.
'has' becomes 'hart' (I can't justify this - if you can you should give a lot more credence to this answer).
'trembling' indicates anagramming the letters.
'inside' indicates putting letters inside.
'fled' anagrammed gives 'dlef'.
'hart' going around 'dlef' is 'HARD LEFT'.
Can you help me to learn more?
(Other definitions for hard left that I've seen before include "Trots, maybe" , "Extreme party people" .)