Crazy, Gershwin accepting allotment by lake (10)
I believe the answer is:
irrational
'crazy' is the definition.
(thesaurus)
'gershwin accepting allotment by lake' is the wordplay.
'gershwin' becomes 'ira' (ira gershwin is george gershwin's brother).
'accepting' indicates putting letters inside.
'allotment' becomes 'ration' (I've seen this in other clues).
'by' says to put letters next to each other.
'lake' becomes 'l'.
'ira' placed around 'ration' is 'irrationa'.
'irrationa'+'l'='IRRATIONAL'
(Other definitions for irrational that I've seen before include "Crazy" , "Illogical" , "Kind of numbers" , "Not logical" , "making no sense" .)