Small building society, say, is barely sufficient (7)
I believe the answer is:
slender
'barely sufficient' is the definition.
(I've seen this before)
'small building society say' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'building society say' becomes 'lender' (a building society lends money).
's'+'lender'='SLENDER'
'is' is the link.
(Other definitions for slender that I've seen before include "Thin or narrow" , "Gracefully thin and elegant" , "Svelte" , "inadequate" , "Gracefully slim" .)