Call to allow lock (7)
I believe the answer is:
ringlet
'lock' is the definition.
(both can mean a piece of someone's hair)
'call to allow' is the wordplay.
'call' becomes 'ring' (telephone call).
'to allow' becomes 'let' ('let' can be a synonym of 'allow').
'ring'+'let'='RINGLET'
(Other definitions for ringlet that I've seen before include "A long tubular curl" , "Handy device" , "Curling lock of hair" , "Hair feature" , "Cluster of hair" .)