Alien found under farm building's thatch? (6)
I believe the answer is:
barnet
'thatch?' is the definition.
(Barnet Fair is Cockney rhyming slang for hair)
'alien found under farm building's' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'found under' says to put letters next to each other.
'farm building' becomes 'barn' (barn is a kind of farm building).
'et' put after 'barn' is 'BARNET'.
(Other definitions for barnet that I've seen before include "Hair (rhyming slang)" , "Locks" , "football team" , "N. London borough" , "London borough, site of 1471 battle" .)