Feel a smaller amount would be foolish (9)
I believe the answer is:
senseless
'foolish' is the definition.
(not sensible)
'feel a smaller amount' is the wordplay.
'feel' becomes 'sense' (synonyms).
'a smaller amount' becomes 'less' (I've seen this before).
'sense'+'less'='SENSELESS'
'would be' is the link.
(Other definitions for senseless that I've seen before include "Illogical; stupid" , "Irrational" , "Not marked by the use of reason" , "Inane" , "Wildly foolish" .)