Don’t start to strain lock (5)
I believe the answer is:
tress
'lock' is the definition.
(both can mean a piece of hair)
'don't start to strain' is the wordplay.
'don't start' means to remove the first letter.
'to strain' becomes 'stress' (stress is a kind of strain).
'stress' with its first letter removed is 'TRESS'.
(Other definitions for tress that I've seen before include "Curl - lock" , "Rests (anag.)" , "Switch" , "Bit of hair" , "Obsolete support" .)