That men act foolishly is fitting (10)
I believe the answer is:
attachment
'fitting' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'that men act foolishly' is the wordplay.
'foolishly' is an anagram indicator.
'that'+'men'+'act'='thatmenact'
'thatmenact' with letters rearranged gives 'ATTACHMENT'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for attachment that I've seen before include "Affinity" , "Fond regard" , "communicated via e-mail" , "Document with e-mail" , "what might accompany 50 [EMAIL]" .)