Granted that a large idea may fall short (8)
I believe the answer is:
although
'granted that' is the definition.
('although' can mean 'granted...')
'a large idea may fall short' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'idea' becomes 'thought' (synonyms).
'may fall short' means to remove the last letter.
'thought' with its final letter taken off is 'though'.
'a'+'l'+'though'='ALTHOUGH'
(Other definitions for although that I've seen before include "Hugo, halt! (anag)" , "Notwithstanding" , "However, but" , "Whilst" , "Despite the fact that (8)" .)