Whimsical nobleman upset by lake (5)
I believe the answer is:
droll
'whimsical' is the definition.
(thesaurus)
'nobleman upset by lake' is the wordplay.
'nobleman' becomes 'lord' ('lord' can be a synonym of 'nobleman'**).
'upset' is a reversal indicator.
'by' means one lot of letters go next to another.
'lake' becomes 'l'.
'lord' in reverse letter order is 'drol'.
'drol'+'l'='DROLL'
(Other definitions for droll that I've seen before include "Humorous or whimsical" , "Zany" , "amusing, in a strange way" , "Oddly amusing or comical" , "Whimsically comical" .)