Intrude by chance or accidentally (8)
I believe the answer is:
encroach
'intrude' is the definition.
(I've seen this before)
'by chance or accidentally' is the wordplay.
'by' means one lot of letters go next to another.
'accidentally' indicates an anagram (a chaotic or accidental ordering of the letters).
'chance'+'or' is 'chanceor'.
'chanceor' with letters rearranged gives 'ENCROACH'.
(Other definitions for encroach that I've seen before include "Intrude on someone's property or time" , "Gradually intrude (upon another's space)" , "Advance beyond proper limit" , "Gradually intrude (on person's area)" , "Tread on someone's toes" .)