Startle with a large member (5)
I believe the answer is:
alarm
'startle' is the definition.
(I've seen this before)
'with a large member' is the wordplay.
'with' means one lot of letters go next to another.
'large' becomes 'L' (eg in clothes sizes).
'member' becomes 'arm' (member can mean a limb).
'a' put next to 'l' is 'al'.
'al'+'arm'='ALARM'
(Other definitions for alarm that I've seen before include "Tocsin" , "Warning sound/device" , "Shock; danger signal" , "A rousing sound in the morning" , "State of (sudden) fear" .)