Most gentle criminal 'Slim Ted' (7)
I believe the answer is:
mildest
'most gentle' is the definition.
('mild' can be similar in meaning to 'gentle')
'criminal slim ted' is the wordplay.
'criminal' indicates an anagram.
'slim'+'ted'='slimted'
'slimted' is an anagram of 'MILDEST'.
(Other definitions for mildest that I've seen before include "Most gentle" , "calmer than all the others" .)