Firm stocking my salmon (4)
I believe the answer is:
coho
'salmon' is the definition.
(type of salmon)
'firm stocking my' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'stocking' indicates putting letters inside ('stock' can mean 'carry' or 'contain').
'my' becomes 'oh' (both are expressions of surprise).
'co' going around 'oh' is 'COHO'.
(Other definitions for coho that I've seen before include "Small north Pacific salmon" , "Fish" .)