Nobleman investing in variable quantities on an annual basis (6)
I believe the answer is:
yearly
'on an annual basis' is the definition.
'nobleman investing in variable quantities' is the wordplay.
'nobleman' becomes 'earl' (earl is a kind of nobleman).
'investing in' indicates putting letters inside.
'variable' becomes 'y' (I've seen this in other clues).
'quantities' becomes 'y' (I can't explain this - if you can you should believe this answer much more).
'y'+'y'='yy'
'earl' going inside 'yy' is 'YEARLY'.
(Other definitions for yearly that I've seen before include "Every 12 months" , "Every 365 or 366 days" , "At annual intervals" , "Unpleasant noise" , "Annual periodical" .)