Loan shark, unspeakable blackguard first of all (3)

I believe the answer is:

sub

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

'shark unspeakable blackguard first of all' is the wordplay.
'first of all' suggests taking the first letters.
The first letters of 'shark unspeakable blackguard' is 'SUB'.

(Other definitions for sub that I've seen before include "Underwater boat" , "Junior editor" , "Beneath on bus" , "Prefix meaning beneath" , "Loan" .)

I've seen this clue in The Times.

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