Solitary old liberal found in US city (4)
I believe the answer is:
only
'solitary' is the definition.
(I know that solitary can be written as only)
'old liberal found in us city' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'liberal' becomes 'l' (abbreviation).
'found in' indicates putting letters inside.
'us city' becomes 'ny' (short for New York).
'l' inserted into 'ny' is 'nly'.
'o'+'nly'='ONLY'
(Other definitions for only that I've seen before include "And no one else" , "Merely; single" , "And nothing more; sole" , "Single, solitary" , "Simply" .)