Impractical person is absorbed by silly detail (8)
I believe the answer is:
idealist
'impractical person' is the definition.
(I've seen this before)
'is absorbed by silly detail' is the wordplay.
'absorbed by' indicates putting letters inside.
'silly' indicates an anagram.
'detail' is an anagram of 'idealt'.
'is' going into 'idealt' is 'IDEALIST'.
(Other definitions for idealist that I've seen before include "One who aspires to perfect standards" , "Dreamer" , "One has high hopes" , "man of principle" , "Detail is about person with aspirations toward perfection (8)" .)