Fool takes on leading German – it shouldn’t last long (4,3)
I believe the answer is:
stop gap
'it shouldn't last long' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'fool takes on leading german' is the wordplay.
'fool' becomes 'sap' (synonyms).
'takes' means one lot of letters goes inside another.
'on leading' becomes 'top' (both can mean first-class. I am not sure about the 'on' bit.).
'german' becomes 'g' (abbreviation).
'top'+'g'='topg'
'sap' going around 'topg' is 'STOP-GAP'.
Can you help me to learn more?
(Other definitions for stop gap that I've seen before include "Expedient" , "stretch between two sentences?" , "Interim" .)