One on outskirts of Liverpool is ailing (3)
I believe the answer is:
ill
'is ailing' is the definition.
(similar in meaning)
'one on outskirts of liverpool' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'outskirts of liverpool' becomes 'LL' (first and last letters of 'Liverpool').
'i'+'ll'='ILL'
(Other definitions for ill that I've seen before include "Not well, sick" , "Shakespeare: '. . . met by moonlight'" , "Dicky" , "Where Chicagoans are briefly" , "Harm" .)