Short, endlessly short and unwelcome occupants (9)
I believe the answer is:
squatters
'unwelcome occupants' is the definition.
(a squatter occupies a building without permission)
'short endlessly short and' is the wordplay.
'short' becomes 'squatty' (similar in meaning).
'endlessly' means to remove the last letter.
'short and' becomes 'ers' (I can't explain this - if you can you should give a lot more credence to this answer).
'squatty' with its last letter taken off is 'squatt'.
'squatt'+'ers'='SQUATTERS'
(Another definition for squatters that I've seen is " Illegal occupiers".)