What allows boats to advance forward? (4)
I believe the answer is:
lock
'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" .)