Makes watertight, say (5)
I believe the answer is:
seals
'makes watertight say' is the definition.
(making watertight is a kind of sealing)
This is all the clue.
(Other definitions for seals that I've seen before include "Fastens or closes securely" , "Marine mammals" , "Great swimmers" , "They make an impression" , "Makes waterproof" .)