Swimmer making money one paper's written about inside (7)
I believe the answer is:
catfish
'swimmer' is the definition.
(catfish is a kind of aquatic vertebrate)
'money one paper's written about inside' is the wordplay.
'money' becomes 'cash' (I've seen this before**).
'one' becomes 'i' (Roman numeral).
'paper' becomes 'ft'.
'written about' says the letters should be written in reverse.
'inside' means one lot of letters goes inside another.
'i'+'ft'='ift'
'ift' reversed gives 'tfi'.
'cash' placed around 'tfi' is 'CATFISH'.
'making' is the link.
(Other definitions for catfish that I've seen before include "Swimmer with whiskery barbels" , "Freshwater type" , "His fact (anag)" , "Bewhiskered swimmer?" , "Aquatic creature with barbels" .)