Says sees grab (5)
I believe the answer is:
seize
'grab' is the definition.
(I know that grab is a kind of seize)
'says sees' is the wordplay.
'says' indicates a 'sounds like' (homophone) clue.
'sees' sounds like 'SEIZE'.
(Other definitions for seize that I've seen before include "Grasp hold of" , "Grab suddenly" , "a sweet number in Grenoble, say" , "Suddenly take hold of" , "Grasp forcibly" .)