Confuse by removing top of oar (5)
I believe the answer is:
addle
'confuse' is the definition.
(I know that addle is a more specific form of the action confuse)
'removing top of oar' is the wordplay.
'removing top of' means to remove the first letter (I've seen 'removes top' mean this).
'oar' becomes 'paddle' (paddle is a kind of oar).
'paddle' with its initial letter removed is 'ADDLE'.
'by' is the link.
(Other definitions for addle that I've seen before include "(Of eggs) turn bad" , "becomes muddled" , "Get confused" , "Go rotten" , "Confuse - decay" .)