Throw drunkard over top of seawall (4)
I believe the answer is:
toss
'throw' is the definition.
(tossing is a kind of throwing)
'drunkard over top of seawall' is the wordplay.
'drunkard' becomes 'sot' (synonyms).
'over' says the letters should be written backwards.
'top of' indicates taking the first letters.
The initial letter of 'seawall' is 's'.
'sot' back-to-front is 'tos'.
'tos'+'s'='TOSS'
(Other definitions for toss that I've seen before include "Throw carelessly" , "Leave decision to chance" , "Casual throw" , "ball" , "Fling" .)