Lock phone then check (7)
I believe the answer is:
ringlet
'lock' is the definition.
(both can mean a piece of someone's hair)
'phone then check' is the wordplay.
'phone' becomes 'ring' (to ring on the phone).
'then' means one lot of letters go next to another.
'check' becomes 'let' (I've seen this in other clues).
'ring'+'let'='RINGLET'
(Other definitions for ringlet that I've seen before include "Curling lock of hair" , "Curled lock of hair" , "Handy device" , "Little curl" , "Bit of hair" .)