Flash one's lower parts (7)
I believe the answer is:
secondi
'lower parts' is the definition.
(I've seen this before)
'flash one's' is the wordplay.
'flash' becomes 'second' (both can mean a very short amount of time).
'one' becomes 'i' (Roman numeral).
'second'+'i'='SECONDI'
(Other definitions for secondi that I've seen before include "Bass parts" , "Parts of duet" , "Lower parts" .)