One who imagines red mare running around (7)
I believe the answer is:
dreamer
'one who imagines' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'red mare running around' is the wordplay.
'running around' indicates anagramming the letters.
'red'+'mare'='redmare'
'redmare' with letters rearranged gives 'DREAMER'.
Can you help me to learn more?
(Other definitions for dreamer that I've seen before include "One fantasising" , "One lost in abstraction" , "One imagines" , "such an unpractical person!" , "Martin Luther King" .)