Silly to be in an open elevator (5)
I believe the answer is:
inane
'silly' is the definition.
(inane means silly or foolish)
'in an open elevator' is the wordplay.
'open' suggests taking the first letters (take the opening letter).
The initial letter of 'elevator' is 'e'.
'in'+'an'+'e'='INANE'
'to be' is the link.
(Other definitions for inane that I've seen before include "Silly, fatuous" , "Without meaning or point" , "unimaginative" , "Silly, witless" , "Unintelligent" .)