My investment company's base (1,3)
I believe the answer is:
i say
'my' is the definition.
(both are exclamations of shock)
'investment company's base' is the wordplay.
'investment' becomes 'isa' (Individual Savings Account).
'base' suggests the final letters (the base of the word).
The last letter of 'company' is 'y'.
'isa'+'y'='I SAY!'
(Other definitions for i say that I've seen before include "Exclamation of surprise" , "Look here; oh, my" , "Gosh!" , "Dear me!" , "Expression of surprise" .)