Shark tail is seen within banks of Forth (7)
I believe the answer is:
dogfish
'shark' is the definition.
(dogfish is a kind of shark)
'tail is seen within banks of forth' is the wordplay.
'tail' becomes 'dog' (both can mean to follow).
'seen within' means one lot of letters goes inside another.
'banks of' says to hollow out the word (remove centre letters) (the banks are the sides of the word).
'forth' with its middle taken out is 'fh'.
'is' put into 'fh' is 'fish'.
'dog'+'fish'='DOGFISH'
(Other definitions for dogfish that I've seen before include "'Small shark, sounds canine (7)'" , "Small, edible shark" , "Restaurants call it rock salmon" , "Nursehound" .)