Speed at which the smack sails? (8,4)

'smack sails?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'speed at which' is the wordplay.
'speed' becomes 'pace' (pace is a kind of speed).
'at' says to put letters next to each other (I've seen this in other clues).
'which' becomes 'spanking' (I can't justify this - if you can you should give a lot more credence to this answer).
'pace' put after 'spanking' is 'SPANKING PACE'.

'the' is the link.

Can you help me to learn more?