Operating system also free - there's nothing in it (7)
I believe the answer is:
android
'operating system' is the definition.
(Android is an operating system for mobile phones)
'also free there's nothing in it' is the wordplay.
'also' becomes 'and' (both can mean in addition).
'free' becomes 'rid' (to be free of something is to be rid of it).
'there's nothing' becomes 'o' (I've seen this in other clues).
'in it' indicates putting letters inside.
'rid' going around 'o' is 'roid'.
'and'+'roid'='ANDROID'
(Other definitions for android that I've seen before include "Mechanical man" , "Human-type robot" , "Half man, half machine" , "Humanoid robot" , "Operating system for mobile devices" .)