Every so often reveal catch (3)
I believe the answer is:
eel
'catch' is the definition.
(potentially makes up a fisherman's catch)
'every so often reveal' is the wordplay.
'every so often' means one should take alternating letters.
The alternate letters of 'reveal' are 'EEL'.
(Other definitions for eel that I've seen before include "elusive person" , "Slithery fish" , "Proverbially slippery fish" , "Unagi, in a sushi bar" , "Elongated fish" .)