Lock phone given rental contract (7)
I believe the answer is:
ringlet
'lock' is the definition.
(both can mean a piece of someone's hair)
'phone given rental contract' is the wordplay.
'phone' becomes 'ring' (synonyms).
'given rental contract' becomes 'let' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ring'+'let'='RINGLET'
Can you help me to learn more?
(Other definitions for ringlet that I've seen before include "sort of lock" , "Hair feature" , "Curling lock of hair" , "twist of sorts?" , "Curl, of an Irish dancer say" .)