One understanding one’s time as a dreamer (8)
I believe the answer is:
idealist
'a dreamer' is the definition.
('idealist' can be a synonym of 'dreamer')
'one understanding one's time' is the wordplay.
'one' becomes 'I' (Roman numeral).
'understanding' becomes 'deal' (deal is a kind of understanding).
'one's' becomes 'I's' (Roman numeral).
'time' becomes 't' (abbreviation).
'i'+'deal'+'is'+'t'='IDEALIST'
'as' acts as a link.
(Other definitions for idealist that I've seen before include "Details I accumulated about person with best intentions" , "man of principle" , "Person with lofty and unselfish aspirations" , "Visionary" , "philosopher" .)