My Lord has invested currency -- big fish (7)
I believe the answer is:
grandee
'big fish' is the definition.
(both can mean an important person)
'my lord has invested currency' is the wordplay.
'my lord' becomes 'gee' (both are exclamations of surprise).
'has invested' is an insertion indicator.
'currency' becomes 'rand' (I've seen this before).
'gee' going around 'rand' is 'GRANDEE'.
(Other definitions for grandee that I've seen before include "Person of eminence" , "Angered over Spanish noble" , "VIP" , "Person of high rank" , "Eminent man" .)