Promise oddly glum American organised support (9)
I believe the answer is:
guarantee
'promise' is the definition.
(guaranteeing is a kind of promising)
'oddly glum american organised support' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'organised' becomes 'ran' (I've seen this before).
'support' becomes 'tee' (tee is a kind of support).
The alternate letters of 'glum' are 'gu'.
'gu'+'a'+'ran'+'tee'='GUARANTEE'
(Other definitions for guarantee that I've seen before include "Provide financial security for" , "Formal assurance" , "Assurance, warranty" , "Promising" , "Warranty, pledge" .)