Unrealistic question by one alien losing head (8)
I believe the answer is:
quixotic
'unrealistic' is the definition.
(synonyms)
'question by one alien losing head' is the wordplay.
'question by' becomes 'qu' (abbreviation. I am not sure about the 'by' bit.).
'one' becomes 'i' (Roman numeral).
'alien' becomes 'exotic' (synonyms).
'losing head' means to remove the first letter.
'exotic' with its initial letter removed is 'xotic'.
'qu'+'i'+'xotic'='QUIXOTIC'
(Other definitions for quixotic that I've seen before include "Fancifully impractical" , "Impractically idealistic" , "donnish?" , "Romantically impractical" , "Extravagantly chivalrous like Cerantes' Don" .)