Milestone parties most common in Hertfordshire market town (10)
I believe the answer is:
leominster
'market town' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I can't see?
'milestone parties most common in hertfordshire' is the wordplay.
'parties' is an anagram indicator (the letters move or party around).
'most common in hertfordshire' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'milestone' with letters rearranged gives 'leominste'.
'leominste'+'r'='LEOMINSTER'
Can you help me to learn more?
(Another definition for leominster that I've seen is " Market town in Herefordshire".)