Look at an aquatic predator? One takes excessive interest (4,5)
I believe the answer is:
loan shark
'one takes excessive interest' is the definition.
(a loan shark charges high interest)
'look at an aquatic predator?' is the wordplay.
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'at' says to put letters next to each other.
'aquatic predator?' becomes 'shark' (I've seen this in another clue).
'lo'+'an'+'shark'='LOAN SHARK'
(Other definitions for loan shark that I've seen before include "Financial dealer" , "Usurer" , "One lends money at excessive rates of interest" , "Money lender" , "Unscrupulous moneylender (colloq.)" .)