Mash of poorest quality, but not quite (7)
I believe the answer is:
tatties
'mash of poorest' is the definition.
I can't tell whether this definition defines the answer.
'quality but not quite' is the wordplay.
I cannot really see how this works, but
'quality' could be 'tat' (tat is a kind of quality) and 'tat' is present in the answer.
The remaining letters 'ties' 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?
(Another definition for tatties that I've seen is " things growing in the ground".)