Lose sense from queasiness? As if! (5)
I believe the answer is:
quasi
'as if' is the definition.
(I've seen this in another clue)
'lose sense from queasiness?' is the wordplay.
'lose' indicates anagramming the letters (I've seen 'lost' mean this).
'from' indicates named letters should be taken away.
'queasiness' with 'sense' taken away is 'quais'.
'quais' is an anagram of 'QUASI'.
(Other definitions for quasi that I've seen before include "Partly, semi-" , "Prefix meaning as if, seeming to be but not really" , "Seemingly" , "Partly, to some degree" .)