Jack volunteers to be ringleader (3)
I believe the answer is:
tar
'jack' is the definition.
(thesaurus)
'volunteers to be ringleader' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'to be ringleader' becomes 'r' (first letter of ring).
'ta'+'r'='TAR'
(Other definitions for tar that I've seen before include "Kind of rat for sailor" , "Smoker's black deposit" , "Road-making substance" , "Sticky substance used in road construction" , "Informal name for sailor" .)