Stupid alien in copse (7)
I believe the answer is:
thicket
'copse' is the definition.
('thicket' can be a synonym of 'copse')
'stupid alien' is the wordplay.
'stupid' becomes 'thick' ('thick' can be similar in meaning to 'stupid').
'alien' becomes 'et' (extra terrestrial).
'thick'+'et'='THICKET'
'in' acts as a link.
(Other definitions for thicket that I've seen before include "close gathering of elders?" , "Mass of shrubs and bushes growing close together" , "Dense growth of bushes" , "Coppice" , "Dense bushy area" .)