Was it besieged by some troops with a Yorkist leader? (4)
I believe the answer is:
troy
'was it besieged' is the definition.
(city besieged in classical myth)
'some troops with a yorkist leader?' is the wordplay.
'some' indicates taking the first letters.
'with' means one lot of letters go next to another.
'a yorkist leader?' becomes 'ROY' (abbreviation for Richard of York).
The first letter of 'troops' is 't'.
't'+'roy'='TROY'
'by' acts as a link.
(Other definitions for troy that I've seen before include "Wooden horse siege city" , "Legendary ancient city of northwest Anatolia" , "Besieged city which fell to Wooden Horse" , "Ancient city of the Iliad" , "Ancient city in Asia Minor" .)