Reforming Temperance League, at first secured successor (11)
I believe the answer is:
replacement
'successor' is the definition.
(I know that successor can be written as replacement)
'reforming temperance league at first secured' is the wordplay.
'reforming' indicates an anagram.
'at first' suggests taking the first letters.
'secured' indicates putting letters inside.
The initial letter of 'league' is 'l'.
'temperance' anagrammed gives 'repacement'.
'repacement' placed around 'l' is 'REPLACEMENT'.
(Other definitions for replacement that I've seen before include "sub" , "One superseding another" , "change" , "A substitute item" , "successor?" .)