Missing suspect's first to stop a criminal (6)
I believe the answer is:
absent
'missing' is the definition.
(similar in meaning)
'suspect's first to stop a criminal' is the wordplay.
'suspect's first' becomes 's' (1st letter of 'suspect').
'to stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'criminal' becomes 'bent' (bent can mean corrupt or criminal).
'a'+'bent'='abent'
's' put into 'abent' is 'ABSENT'.
(Other definitions for absent that I've seen before include "nowhere to be seen" , "lack of attention" , "Not present or focused" , "Missing, lacking" , "Not where one should be" .)