Once viewed old things in another way (2,5)
I believe the answer is:
on sight
'once viewed' is the definition.
(I've seen this before)
'old things in another way' is the wordplay.
I cannot quite see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
an anagram of 'things' is 'nsight' which is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for on sight that I've seen before include "at time of presentation" , "Once viewed" , "How good musicians can play" , "when observed" , "How to shoot dangerous criminal" .)