Go off and swim without a top (5)
I believe the answer is:
addle
'go off' is the definition.
(eg an egg might addle)
'swim without a top' is the wordplay.
'swim' becomes 'paddle' (paddling is a kind of swimming).
'without a top' means to remove the first letter (in a down clue, top can mean the top letter).
'paddle' with its first letter taken off is 'ADDLE'.
'and' acts as a link.
(Other definitions for addle that I've seen before include "(Of eggs) become rotten" , "Like a bad egg" , "Muddle or confuse" , "Rot" , "Scramble" .)