Pennies from heaven (4)
I believe the answer is:
rain
'heaven' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'pennies from' is the wordplay.
'pennies' becomes 'ra' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'from' becomes 'in' (I've seen this in other clues).
'ra'+'in'='RAIN'
Can you help me to learn more?
(Other definitions for rain that I've seen before include "Wet weather" , "downfall?" , "Fall wetly" , "Come down in showers" , "Pelt" .)