Hypothesis that has no alternative for these people (4)
I believe the answer is:
they
'these people' is the definition.
(they refers to a group of people)
'hypothesis that has no alternative' is the wordplay.
'hypothesis' becomes 'theory' (synonyms).
'that has no' is a deletion indicator.
'alternative' becomes 'or'.
'theory' with 'or' taken away is 'THEY'.
'for' acts as a link.
(Other definitions for they that I've seen before include "Third person plural" , "People generally" , "Do -- Know It's Christmas (Band Aid)" , "Those people (vb. subject)" , "Pronoun for more than one" .)