Statement dismissing the relative (3)
I believe the answer is:
sis
'relative' is the definition.
(short for sister)
'statement dismissing the' is the wordplay.
'statement' becomes 'thesis' (statement of an argument).
'dismissing' indicates named letters should be taken away.
'thesis' with 'the' removed is 'SIS'.
(Other definitions for sis that I've seen before include "Sibling (informal)" , "relative" , "Secret Intelligence Service (abbr.)" , "Girl" , "spies ..." .)