A ringleader captured by police is bitter (5)
I believe the answer is:
acrid
'is bitter' is the definition.
(I've seen this before)
'a ringleader captured by police' is the wordplay.
'ringleader' becomes 'r' (first letter of ring).
'captured by' means one lot of letters goes inside another.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'r' going into 'cid' is 'crid'.
'a'+'crid'='ACRID'
(Other definitions for acrid that I've seen before include "Pungent or sharp, like smoke" , "Sour, pungent" , "irritating" , "Unpleasantly pungent" , "Having a bitter taste or small, like smoke" .)