Sailors wasting five years at sea? They probably won't agree! (8)
I believe the answer is:
naysayer
'sea? they probably won't agree' is the definition.
The definition suggests an adverb but the answer is not.
'sailors wasting five years' is the wordplay.
'sailors' becomes 'navy' (group of sailors).
'wasting' is a deletion indicator.
'five' becomes 'V' (Roman numeral).
'years' becomes 'sayer' (I can't explain this - if you can you should give a lot more credence to this answer).
'navy' with 'v' removed is 'nay'.
'nay'+'sayer'='NAYSAYER'
'at' is the link.
Can you help me to learn more?
(Other definitions for naysayer that I've seen before include "One who denies" , "Yes, Aryan may be person who habitually denies or refuses" , "He refuses to agree" , "Obstructionist" .)