Closely examine small prison (4)
I believe the answer is:
scan
'closely examine' is the definition.
(to scan is to examine in detail)
'small prison' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'prison' becomes 'can' (the can is American slang for prison).
's'+'can'='SCAN'
(Other definitions for scan that I've seen before include "Hercule ---, detective" , "Medical examination" , "Look over (in order to copy?)" , "Scrutinise; glance over" , "Read hastily" .)