Harm dodged by one sailor that’s turned up a snake (5)
I believe the answer is:
mamba
'a snake' is the definition.
(I know that Mamba is a type of elapid)
'harm dodged by one sailor that's turned up' is the wordplay.
'harm' becomes 'maim' (to maim is to harm or injure).
'dodged by' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'sailor' becomes 'ab' (abbreviation for able seaman).
'that's turned up' is a reversal indicator (in a down clue, letters go up).
'maim' with 'i' taken out is 'mam'.
'ab' written backwards gives 'ba'.
'mam'+'ba'='MAMBA'
(Other definitions for mamba that I've seen before include "Venomous African tree snake" , "Deadly African snake" , "Snake that might kill" , "Poisonous creature" , "Venomous Afr. snake" .)