Docile old criminal welcomes end (8)
I believe the answer is:
obedient
'docile' is the definition.
('obedient' can be similar in meaning to 'docile')
'old criminal welcomes end' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'criminal' becomes 'bent' (I've seen this before).
'welcomes' indicates putting letters inside (in sense of 'invites in').
'end' becomes 'die'.
'bent' placed around 'die' is 'bedient'.
'o'+'bedient'='OBEDIENT'
(Other definitions for obedient that I've seen before include "A good dog should be" , "Docile" , "Like monk" , "Willing to comply" , "Following instructions" .)