Pub included by the ringleader, slimmer (7)
I believe the answer is:
thinner
'slimmer' is the definition.
('thin' can be similar in meaning to 'slim')
'pub included by the ringleader' is the wordplay.
'pub' becomes 'inn' (an inn is a type of pub).
'included by' is an insertion indicator.
'ringleader' becomes 'r' (first letter of ring).
'inn' placed inside 'the' is 'thinne'.
'thinne'+'r'='THINNER'
(Other definitions for thinner that I've seen before include "More flimsy" , "Diluting solvent" , "Finer; turps, e.g." , "slimmers hope to get" , "Dissoving agent" .)