Such an invaluable assistant is just present (5,4)
I believe the answer is:
right hand
'such an' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'invaluable assistant is just present' is the wordplay.
I cannot quite see how this works, but
'assistant' could be 'hand' (hand can mean a worker or employee) and 'hand' is found in the answer.
'just' could be 'right' (similar in meaning) and 'right' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for right hand that I've seen before include "description of 1 across [ASSISSTANT] perhaps" , "stalwart" , "Such a helpful man" , "to be relied on" , "Indispensable assistant" .)