Crowded hospital beset by parasite (5)
I believe the answer is:
thick
'crowded' is the definition.
(I've seen this before)
'hospital beset by parasite' is the wordplay.
'hospital' becomes 'h' (abbreviation for hospital on maps).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'parasite' becomes 'tick' (type of parasite).
'h' inserted inside 'tick' is 'THICK'.
(Other definitions for thick that I've seen before include "Crowded" , "deficient in understanding" , "Concentrated" , "Stupid (as two short planks?)" , "Stupid; close-packed" .)