Small time criminal cases rear of factory block (6)
I believe the answer is:
stymie
'block' is the definition.
('stymie' can be a synonym of 'block')
'small time criminal cases rear of factory' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'criminal' indicates an anagram.
'cases' means one lot of letters goes inside another.
'rear of' says to take the final letters.
The final letter of 'factory' is 'y'.
'time' is an anagram of 'tmie'.
'tmie' placed around 'y' is 'tymie'.
's'+'tymie'='STYMIE'
(Other definitions for stymie that I've seen before include "Thwart - block - my site (anag)" , "balk" , "Block or thwart an activity" , "Block; hinder" , "Block or thwart, say a golf putt" .)