Dreadful crustacean in river drained lake (9)
I believe the answer is:
execrable
'dreadful' is the definition.
(I've seen this before)
'crustacean in river drained lake' is the wordplay.
'crustacean' becomes 'crab' (crab is a kind of crustacean).
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'drained' says to hollow out the word (remove centre letters).
'lake' with its middle removed is 'le'.
'exe'+'le'='exele'
'crab' put inside 'exele' is 'EXECRABLE'.
(Other definitions for execrable that I've seen before include "Awful" , "Appallingly bad" , "Abominable, detestable" , "Loathsome" , "Woeful" .)