High land in Thames area (4)
I believe the answer is:
mesa
'high land' is the definition.
(I've seen this before)
'in thames area' is the wordplay.
'in' indicates a hidden word.
'MESA' is hidden in the letters of 'thames area'.
(Other definitions for mesa that I've seen before include "Flat-topped hill with steep sides, common in the southwestern US" , "Isolated flat-topped hill" , "Flat-topped mountain" , "Flat tableland with steep edges" , "US steep-sided tableland" .)