Lock where key may be found a hindrance (7)
I believe the answer is:
ringlet
'lock' is the definition.
(both can mean a piece of someone's hair)
'where key may be found a hindrance' is the wordplay.
'where key may be found' becomes 'ring' (as in a key ring).
'a hindrance' becomes 'let' (I've seen this in other clues).
'ring'+'let'='RINGLET'
(Other definitions for ringlet that I've seen before include "Curl, of an Irish dancer say" , "Handy device" , "A curl - kind of tingler" , "Bit of hair" , "hair styling" .)