Syndicate with one pound investment maturing (8)
I believe the answer is:
ripening
'maturing' is the definition.
(synonyms)
'syndicate with one pound investment' is the wordplay.
'syndicate' becomes 'ring' (I've seen this in other clues).
'with' indicates putting letters inside.
'one pound' becomes 'pen' (I've seen this in other clues).
'investment' becomes 'i' (this might be a standard abbreviation I've not previously seen).
'pen'+'i'='peni'
'ring' going around 'peni' is 'RIPENING'.
(Another definition for ripening that I've seen is " Developing, becoming mature".)