Soil of an imaginary shire? That's rich! (4)
I believe the answer is:
loam
'soil' is the definition.
(loam is a kind of soil)
'an imaginary shire? that's rich' is the wordplay.
I cannot really understand how this works, but
'an' could be 'a' and 'a' is located in the answer.
The remaining letters 'lom' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for loam that I've seen before include "Type of soil" , "Rich soil, good for growing" , "A rich fertile soil" , "Rich soil containing decayed matter" , "Rich soil with decayed vegetable matter" .)