The Roman Map of Britain Cleuionisso statio Topsham, Devon
Deuionisso (R&C 10) next
is painful to admit that this baffled me for over ten years. The initial d-form
is the result of an aggregated cl, otherwise the name seems intact.
Topsham was a military supply-base on the east bank of the river Exe at the mouth of the river Clyst (SX9688). Ekwall derives Clyst from the IE root *kleu- 'to wash, to swill' found in the Scottish Clyde and the Welsh Clydachs. Mills has Clyst as probably meaning 'sea inlet'. *Cleuion- would be 'kleu-i-stream', and -isso 'sense of place'. Topsham was the port for Isca Exeter (R&C 26), thus the Latin term statio.
This concludes one of the longer sequences, Dorchester - Topsham along Margary 4.
There is a lost Cornish Kaerclewent - Padel, O.J. (1985) Cornish place-name elements, p. 53.
Clauinio/Clausentum (Bitterne) has some potential for a relationship to *kleu-, particularly if both are slightly corrupt versions of *Cleuento.