Crossword Genius

What allows boats to advance forward? (4)

Ross

I believe the answer is:

lock

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'forward?' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for lock that I've seen before include "part of 10 [CANAL]" , "Fasten securely" , "Door fastener or tuft of hair" , "Security device - tuft of hair" , "Canal feature - hair piece" .)

I've seen this clue in The Telegraph.
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