Stupidly dock at point under bottom of Thames Barrier (8)
I believe the answer is:
stockade
'barrier' is the definition.
(thesaurus)
'stupidly dock at point under bottom of thames' is the wordplay.
'stupidly' indicates anagramming the letters.
'point' becomes 'e' (E is an example**).
'under' means one lot of letters go next to another.
'bottom of' suggests the final letters.
The final letter of 'thames' is 's'.
'dock'+'at'='dockat'
'dockat' with letters rearranged gives 'tockad'.
'tockad'+'e'='tockade'
'tockade' after 's' is 'STOCKADE'.
(Other definitions for stockade that I've seen before include "military prison" , "Staked barrier" , "Fortification" , "defensive structure" , "Barrier of stakes and timbers" .)