Its harvest is well gathered, no doubt (8)
I believe the answer is:
oilfield
'no doubt' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'its harvest is well gathered' is the wordplay.
I cannot quite see how this works, but
'well' could be 'oil' (oil well is a kind of well) and 'oil' is present in the answer.
The remaining letters 'field' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oilfield that I've seen before include "Area of liquid fuel reserves" , "Source of petroleum" .)