Loan for boat (3)

I believe the answer is:

sub

I believe this is a double definition.

'loan' is the first definition.
(I've seen this before)

'for boat' is the second definition.
(submarine)

(Other definitions for sub that I've seen before include "Contribution; a boat" , "Prefix meaning under or lower" , "Warship; membership fee" , "Navy vessel" , "Prefix meaning beneath" .)

I've seen this clue in The Times.

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