It's said to have an arresting effect on sailors (5)

'effect on sailors' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'it's said to have an arresting' is the wordplay.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'said' becomes 'bly' (I can't justify this - if you can you should give a lot more credence to this answer).
'to have' means one lot of letters goes inside another.
'an' becomes 'a'.
'arresting' is an insertion indicator.
'bly' placed around 'a' is 'blay'.
'e' inserted inside 'blay' is 'BELAY'.

Can you help me to learn more?