Good, if backed in money, for a shark (7)
I believe the answer is:
dogfish
'a shark' is the definition.
(dogfish is a kind of shark)
'good if backed in money' is the wordplay.
'good' becomes 'g' (abbreviation).
'backed' says the letters should be written in reverse.
'in' indicates putting letters inside.
'money' becomes 'dosh' (I've seen this before**).
'if' back-to-front is 'fi'.
'g'+'fi'='gfi'
'gfi' going inside 'dosh' is 'DOGFISH'.
'for' acts as a link.
(Other definitions for dogfish that I've seen before include "'Small shark, sounds canine (7)'" , "Nursehound" , "Restaurants call it rock salmon" , "Small, edible shark" .)