Unconvincing leader of gang's obsession (5)
I believe the answer is:
thing
'obsession' is the definition.
(thesaurus)
'unconvincing leader of gang's' is the wordplay.
'unconvincing' becomes 'thin' (I've seen this before).
'leader of' says to take the initial letters.
The first letter of 'gangs' is 'g'.
'thin'+'g'='THING'
(Other definitions for thing that I've seen before include "An unnamed object or item" , "cup of tea" , "Unspecified object" , "Body" , "Individual unit" .)