Coop up in it, which is foolish (5)
I believe the answer is:
inept
'is foolish' is the definition.
(thesaurus)
'coop up in it' is the wordplay.
'coop' becomes 'pen' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' indicates putting letters inside.
'pen' backwards is 'nep'.
'nep' put inside 'it' is 'INEPT'.
'which' acts as a link.
(Other definitions for inept that I've seen before include "Clumsy - silly" , "lacks skill" , "Not competent" , "Standard required in a piece of work (colloq.)" , "Foolish" .)