Unlimited entry hacking into smart key (7)
I believe the answer is:
control
'key' is the definition.
(I know that Control is a type of modifier key)
'unlimited entry hacking into smart' is the wordplay.
'unlimited' indicates the central letters (discard the edges or 'limits' of the word).
'hacking into' indicates putting letters inside.
'smart' becomes 'cool'.
The middle letters of 'entry' are 'ntr'.
'ntr' put within 'cool' is 'CONTROL'.
(Other definitions for control that I've seen before include "Rule" , "Restraint" , "Keep in line" , "Influencing power" , "Rein" .)