The old Republican is beginning to trust no practical person? (8)
I believe the answer is:
theorist
'no practical person?' is the definition.
(theory rather than practice)
'the old republican is beginning to trust' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'republican' becomes 'r' (abbreviation).
'beginning to' says to take the initial letters.
The initial letter of 'trust' is 't'.
'the'+'o'+'r'+'is'+'t'='THEORIST'
(Other definitions for theorist that I've seen before include "Expert in established hypotheses" , "Thinker" , "Speculator" , "An expert in abstract principles" , "Creator of hypothesis" .)