... one that turns round at a Welsh town (5)
I believe the answer is:
neath
'a welsh town' is the definition.
(I've seen this before)
'one that turns round at' is the wordplay.
'one' becomes 'hen' (I can't justify this - if you can you should believe this answer much more).
'that turns' says the letters should be written in reverse.
'round' means one lot of letters goes inside another.
'hen' in reverse letter order is 'neh'.
'neh' going around 'at' is 'NEATH'.
Can you help me to learn more?
(Other definitions for neath that I've seen before include "town in South Wales" , "Under, poetically" , "Lower than - a town in West Glamorgan" , "Welsh industrial town" , "Town near Swansea" .)