Harmless dope let loose (6)
I believe the answer is:
gentle
'harmless' is the definition.
(thesaurus)
'dope let loose' is the wordplay.
'dope' becomes 'gen' (I've seen this in other clues**).
'loose' is an anagram indicator (letters are loosened from their original positions).
'let' is an anagram of 'tle'.
'gen'+'tle'='GENTLE'
(Other definitions for gentle that I've seen before include "amiable" , "that may help angler catch fish" , "Mild and benign" , "'Do not go . . . . . . into that good night' (Dylan Thomas)" , "Kind-natured" .)