One share in decision that’s a surprise (1,3)
I believe the answer is:
i say
'that's a surprise' is the definition.
('I say' is an expression of surprise)
'one share in decision' is the wordplay.
'one' becomes 'I' (Roman numeral).
'share in decision' becomes 'say' (as in having a say in a decision).
'i'+'say'='I SAY!'
(Other definitions for i say that I've seen before include "Dear me!" , "Look here; oh, my" , "That is surprising!" , "Crikey!" , "Comment from protestor" .)