Perhaps fish with feet conform (3,3,4)
I believe the answer is:
toe the line
'conform' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for toe the line that I've seen before include "Follow orders" , "Conform to expected requirements" , "when the heat is on?" , "Conform to expectations" , "Comply with authority" .)