An entrance without it causes stir (7)
I believe the answer is:
agitate
'stir' is the definition.
(I know that stir can be written as agitate)
'an entrance without it' is the wordplay.
'an' becomes 'a'.
'entrance' becomes 'gate' (type of entrance).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'gate' enclosing 'it' is 'gitate'.
'a'+'gitate'='AGITATE'
'causes' is the link.
(Other definitions for agitate that I've seen before include "Fluster" , "Stir (up public opinion?)" , "Trouble, make nervous" , "Disturb or stir up" , "Unsettle" .)