Means to imprison ringleader if suspicious (4)
I believe the answer is:
wary
'suspicious' is the definition.
(thesaurus)
'means to imprison ringleader' is the wordplay.
'means' becomes 'way' ('way' can be a synonym of 'means'**).
'to imprison' indicates putting letters inside.
'ringleader' becomes 'r' (first letter of ring).
'way' enclosing 'r' is 'WARY'.
'if' is the link.
(Other definitions for wary that I've seen before include "Cautious, suspicious" , "Careful - cautious" , "Cautious - vigilant" , "Watchful" , "Displaying caution" .)