Intrude in health resort, breaking lock? (8)
I believe the answer is:
trespass
'intrude' is the definition.
('trespass' can be a synonym of 'intrude')
'health resort breaking lock?' is the wordplay.
'health resort' becomes 'spa' (I've seen this before).
'breaking' is an insertion indicator.
'lock?' becomes 'tress' (I've seen this before).
'spa' put within 'tress' is 'TRESPASS'.
'in' acts as a link.
(Other definitions for trespass that I've seen before include "Intrude on another's property" , "Action of intruder" , "Enter the Forbidden Land" , "Illegally entering" , "Transgress" .)