Large wave ruined suit upset man's wearing (7)
I believe the answer is:
tsunami
'large wave' is the definition.
(I've seen this before)
'ruined suit upset man's wearing' is the wordplay.
'ruined' indicates an anagram.
'upset' indicates anagramming the letters.
'wearing' means one lot of letters goes inside another.
'man' is an anagram of 'nam'.
'suit' is an anagram of 'tsui'.
'tsui' going around 'nam' is 'TSUNAMI'.
(Other definitions for tsunami that I've seen before include "Large tidal wave" , "Marine natural disaster" , "High sea wave" , "Huge wave caused by earth tremor" , ""Harbour wave"" .)