Destructive wave knocked over chap in new suit (7)
I believe the answer is:
tsunami
'destructive wave' is the definition.
(I've seen this in another clue)
'knocked over chap in new suit' is the wordplay.
'knocked over' says the letters should be written backwards.
'chap' becomes 'man' (chap can mean a man).
'in' means one lot of letters goes inside another.
'new' is an anagram indicator.
'man' back-to-front is 'nam'.
'suit' anagrammed gives 'tsui'.
'nam' put within 'tsui' is 'TSUNAMI'.
(Other definitions for tsunami that I've seen before include "Strong surge" , "High sea wave" , "Huge wave caused by earth tremor" , ""Harbour wave"" , "Long sea wave" .)