Hospital unit is perfect (4)
I believe the answer is:
hone
'perfect' is the definition.
('hone' can be a synonym of 'perfect')
'hospital unit' is the wordplay.
'hospital' becomes 'h'.
'unit' becomes 'one' ('one' can be a synonym of 'unit'**).
'h'+'one'='HONE'
'is' is the link.
(Other definitions for hone that I've seen before include "Sharpen tool on stone" , "Sharpen (a blade)" , "Smooth stone" , "Strop" , "Polish" .)