A duck, flat out, will certainly not sink! (5)
I believe the answer is:
float
'not sink' is the definition.
('float' is the opposite to 'sink')
'a duck flat out' is the wordplay.
'a duck' becomes 'o' (resembles zero - 'duck' in cricket).
'out' is an insertion indicator.
'o' going inside 'flat' is 'FLOAT'.
'will certainly' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for float that I've seen before include "Be buoyant" , "Bob" , "Lie on the surface of liquid." , "bread supply?" , "Not sink" .)