Wide river restricting swimming in vest (9)
I believe the answer is:
extensive
'wide' is the definition.
(similar in meaning)
'river restricting swimming in vest' is the wordplay.
'river' becomes 'exe' (River Exe).
'restricting' means one lot of letters goes inside another.
'swimming' indicates an anagram (letters swim around into a new order).
'in'+'vest'='invest'
'invest' with letters rearranged gives 'tensiv'.
'exe' enclosing 'tensiv' is 'EXTENSIVE'.
(Other definitions for extensive that I've seen before include "Broad in scope" , "Exist even being wide-ranging" , "Covering a large area" , "Covering a wide area" , "Widespread" .)