Stupid alien in a copse (7)
I believe the answer is:
thicket
'copse' is the definition.
(I know that copse can be written as thicket)
'stupid alien' is the wordplay.
'stupid' becomes 'thick' (similar in meaning).
'alien' becomes 'et' (extra terrestrial).
'thick'+'et'='THICKET'
'in a' acts as a link.
(Other definitions for thicket that I've seen before include "Dense group of trees or bushes" , "Dense mass of trees or shrubs" , "A patch of shrubs and small trees" , "Dense bushy area" , "close gathering of elders?" .)