One thinks of them as abstractions (5)
I believe the answer is:
ideas
'abstractions' is the definition.
(I've seen this in another clue)
'one thinks of them as' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'as' is found in the answer.
The remaining letters 'de' 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 ideas that I've seen before include "Thoughts, perceptions" , "Notions, thoughts" , "Thoughts from Sadie" , "plans" , "Thoughts or opinions" .)