Crime boss, mob leader, ends up captured by fighter (2,3)
I believe the answer is:
mr big
'crime boss' is the definition.
(I've seen this in another clue)
'mob leader ends up captured by fighter' is the wordplay.
'ends' indicates one should take the final letters.
'up' indicates anagramming the letters (letters get chucked up).
'captured by' indicates putting letters inside.
'fighter' becomes 'mig' (fighter jet produced by Russian company MiG).
The last letters of 'mob leader' are 'br'.
'br' is an anagram of 'rb'.
'rb' inserted inside 'mig' is 'MR BIG'.
(Other definitions for mr big that I've seen before include "chief" , "Important man" , "Top man" , "boss" .)