One's foolish when gripped by cards (7)
I believe the answer is:
jackass
'one's foolish' is the definition.
(a jackass is a foolish person)
'when gripped by cards' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'gripped by' indicates putting letters inside.
'cards' becomes 'jacks' (jack is a kind of card).
'as' going inside 'jacks' is 'JACKASS'.
(Other definitions for jackass that I've seen before include "Asinine person" , "Male donkey - fool" , "One may lark about" , "Stupid person" , "Male donkey; idiot" .)