Sort of film about fish (5)
I believe the answer is:
cling
'sort of film' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'about fish' is the wordplay.
'about' becomes 'c' (circa).
'fish' becomes 'ling' (ling is a kind of fish).
'c'+'ling'='CLING'
Can you help me to learn more?
(Other definitions for cling that I've seen before include "Hold on tightly, be dependent" , "Hold on tightly, refuse to abandon" , "Adhere, hold tight" , "Stick - tinkle" , "Stick close (to surface)" .)