Be fooled by the sound of a fourth cascade (4,3)
I believe the answer is:
fall for
'sound of a fourth cascade' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how they can define each other.
'be fooled by' is the wordplay.
'be' becomes 'fall' (falling is a kind of being).
'fooled by' becomes 'for' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fall'+'for'='FALL FOR'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for fall for that I've seen before include "Have romantic feelings for - or be taken in by?" , "Be taken in by - become infatuated with" , "Acquire romantic impulse toward - take a tumble" , "Be taken in by - romantically?" , "Succumb to charms - take a tumble" .)