Require time during unusual dissection of alien (6)
I believe the answer is:
entail
'require' is the definition.
(thesaurus)
'time during unusual dissection of alien' is the wordplay.
'time' becomes 't'.
'during' indicates putting letters inside.
'unusual dissection of' indicates an anagram.
'alien' anagrammed gives 'enail'.
't' put within 'enail' is 'ENTAIL'.
(Other definitions for entail that I've seen before include "Bring on, involve" , "Bring on an inevitable consequence" , "outmoded fashion" , "Cause" , "Late in (anag.)" .)