Illegally enter hydro, breaking lock (8)
I believe the answer is:
trespass
'illegally enter' is the definition.
'hydro breaking lock' is the wordplay.
'hydro' becomes 'spa' (I've seen this before).
'breaking' indicates putting letters inside.
'lock' becomes 'tress' (I've seen this before).
'spa' placed within 'tress' is 'TRESPASS'.
(Other definitions for trespass that I've seen before include "Go out of bounds" , "Intrude, encroach" , "Go where one shouldn't" , "Sin (archaic)" , "Encroachment" .)