So, first, say you are in addition getting an oversupply (7)
I believe the answer is:
surplus
'an oversupply' is the definition.
(I know that oversupply can be written as surplus)
'so first say you are in addition getting' is the wordplay.
'so' becomes 'lu' (I can't justify this - if you can you should give a lot more credence to this answer).
'first say' becomes 's' (lst letter of 'say').
'you are' becomes 'ur' (textspeak abbreviation).
'in addition' becomes 'PS' (postscript written at the end of a letter).
'getting' means one lot of letters goes inside another.
's'+'ur'+'ps'='surps'
'lu' put into 'surps' is 'SURPLUS'.
Can you help me to learn more?
(Other definitions for surplus that I've seen before include "unnecessary" , "(An) oversupply" , "Excess, extra" , "Extra to requirements" , "superfluity" .)