American in hotel twice finished whitewash (4,2)
I believe the answer is:
hush up
'whitewash' is the definition.
(synonyms)
'american in hotel twice finished' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'in' is an insertion indicator.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'twice' shows that letters should be duplicated.
'finished' becomes 'up' (similar in meaning).
'h' doubled is 'hh'.
'us' placed within 'hh' is 'hush'.
'hush'+'up'='HUSH UP'
(Other definitions for hush up that I've seen before include "Cover up, keep quiet" , "Keep silent" , "Stifle, suppress" , "silence" , "Suppress rumours" .)