Mental prospect could be vast one (5)

I believe the answer is:

vista

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'mental prospect' is the definition.
'vista' can be an answer for 'prospect' (synonyms). I'm unsure of the 'mental' bit.

'could be vast one' is the wordplay.
'could be' indicates an anagram.
'one' becomes 'I' (Roman numeral).
'vast'+'i'='vasti'
'vasti' is an anagram of 'VISTA'.

Can you help me to learn more?

(Other definitions for vista that I've seen before include "Prospect, view" , "Outlook" , "Panoramic view" , "One gives a long look" , "Perspective" .)

I've seen this clue in The Irish News.

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