Common sense is all but pointless (4)
I believe the answer is:
nous
'common sense' is the definition.
(nous is a kind of common sense)
'all but pointless' is the wordplay.
'all but' means to remove the last letter ('all but' can mean 'almost' or 'nearly').
'pointless' becomes 'nouse' (I've seen this before).
'nouse' with its final letter taken away is 'NOUS'.
'is' acts as a link.
(Other definitions for nous that I've seen before include "Informal common sense" , "Common sense - intellect" , "Abroad we" , "FT in Paris, say" , "we recognised in Nancy" .)