Most meagre pub in the street (8)
I believe the answer is:
thinnest
'most meagre' is the definition.
'pub in the street' is the wordplay.
'pub' becomes 'inn' (I've seen this before).
'in' indicates putting letters inside.
'street' becomes 'st'.
'inn' placed into 'the' is 'thinne'.
'thinne'+'st'='THINNEST'
(Other definitions for thinnest that I've seen before include "Most fine" , "Most slender as in tenths" , "Narrowest or leanest" , "Superlatively poor" , "Least fat" .)