The fools pop back on board (4)
I believe the answer is:
saps
'the fools' is the definition.
('sap' can be a synonym of 'fool')
'pop back on board' is the wordplay.
'back' indicates one should take the final letters.
'on' indicates putting letters inside.
'board' becomes 'sas' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The final letter of 'pop' is 'p'.
'p' inserted inside 'sas' is 'SAPS'.
Can you help me to learn more?
(Other definitions for saps that I've seen before include "Weakens, undermines" , "trenches" , "Pass around foolish people" , "Enervates in spas" , "Vital juices" .)