Thick resident in sudden seizure (5)
I believe the answer is:
'thick' is the definition.
(I know that thick can be written as dense)
'resident in sudden seizure' is the wordplay.
'resident' indicates the answer is hidden within the clue (I've seen 'resident of' mean this).
'DENSE' can be found hidden inside 'in sudden seizure'.
(Other definitions for dense that I've seen before include "Thick and tightly-packed" , "Closely packed" , "Thick as undergrowth may be" , "Stupid, crowded together" , "Thick, compact or crowded" .)