Intrusions caused by old-fashioned inside lock (10)
I believe the answer is:
trespasses
'intrusions' is the definition.
('trespass' can be a synonym of 'intrusion')
'old-fashioned inside lock' is the wordplay.
'old-fashioned' becomes 'passe' (synonyms).
'inside' is an insertion indicator.
'lock' becomes 'tress' (both can mean a piece of hair).
'passe' going within 'tress' is 'TRESPASSES'.
'caused by' is the link.
(Other definitions for trespasses that I've seen before include "Forgive them" , "may they be forgiven!" , "Unlawfully enters" .)