One who observes criminal a wretch (7)
I believe the answer is:
watcher
'one who observes' is the definition.
(I've seen this before)
'criminal a wretch' is the wordplay.
'criminal' indicates an anagram.
'a'+'wretch'='awretch'
'awretch' is an anagram of 'WATCHER'.
(Other definitions for watcher that I've seen before include "Alert observer" , "One who observes" , "Big Brother?" , "Onlooker" , "I observe" .)