Confuse by removing top of oar (5)
I believe the answer is:
addle
'confuse' is the definition.
(addling is a kind of confusing)
'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 taken off 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" .)