A single voracious fish announced, one demanding huge interest? (4,5)
I believe the answer is:
loan shark
'announced one demanding huge interest?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'a single voracious fish' is the wordplay.
'a single' becomes 'loan' (I am not sure about this - if you are sure you should believe this answer much more).
'voracious fish' becomes 'shark' (sharks are fish).
'loan'+'shark'='LOAN SHARK'
(Other definitions for loan shark that I've seen before include "Financial dealer" , "Unscrupulous moneylender (colloq.)" , "Usurer" , "One lends money at excessive rates of interest" , "Money lender" .)