With pieces to cut, dress is all there (6,6)
I believe the answer is:
compos mentis
'all there' is the definition.
(both can mean mentally sound)
'with pieces to cut dress is' is the wordplay.
I cannot quite see how this works, but
'pieces' could be 'men' (e.g. in chess) and 'men' is located in the answer.
'is' is within the answer.
The remaining letters 'compost' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for compos mentis that I've seen before include "Capable of reasoning" , "Sane" , "Of sound mind (Latin)" .)