Polish hospital close to overcrowded (4)
I believe the answer is:
sand
'polish' is the definition.
(I've seen this before)
'hospital close to overcrowded' is the wordplay.
'hospital' becomes 'san' (short for sanatorium).
'close to' indicates one should take the final letters (the close/ending of).
The final letter of 'overcrowded' is 'd'.
'san'+'d'='SAND'
(Other definitions for sand that I've seen before include "Fine grit" , "Beach susbstance" , "Shore grains" , "Sort of paper, piper, castle or martin" , "Polish" .)