Trespassers stupidly sit under opening in roof (9)
I believe the answer is:
intruders
'trespassers' is the definition.
('intruder' can be a synonym of 'trespasser')
'stupidly sit under opening in roof' is the wordplay.
'stupidly' is an anagram indicator (a stupid ordering of the letters).
'opening in' indicates taking the first letters.
The first letter of 'roof' is 'r'.
'sit'+'under'+'r'='situnderr'
'situnderr' with letters rearranged gives 'INTRUDERS'.
(Other definitions for intruders that I've seen before include "Trespassers" , "gatecrashers" , "Nurse, dirt of the uninvited" .)