Doubling the common fund up (4)
I believe the answer is:
loop
'doubling' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'common fund up' is the wordplay.
'common fund' becomes 'pool' (I've seen this in another clue).
'up' is a reversal indicator (in down clue: letters go upwards).
'pool' back-to-front is 'LOOP'.
'the' is the link.
Can you help me to learn more?
(Other definitions for loop that I've seen before include "Shape formed by a closed curve that does not intersect itself" , "(Of aircraft) make a circle vertically" , "ring road, say" , "Shape produced by a curve crossing itself" , "Oval shape" .)