Salmon, company reared on island in case of shortage (7)
I believe the answer is:
sockeye
'salmon' is the definition.
(I know that sockeye is a type of salmon)
'company reared on island in case of shortage' is the wordplay.
'company' becomes 'co' (abbreviation).
'reared' is a reversal indicator (in a down clue, letters 'rear up').
'on island' becomes 'key' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'shortage' with its middle removed is 'se'.
'co' in reverse letter order is 'oc'.
'oc'+'key'='ockey'
'ockey' inserted into 'se' is 'SOCKEYE'.
Can you help me to learn more?
(Other definitions for sockeye that I've seen before include "Breed of salmon" , "Commercially important salmon" , "Blue-backed salmon" , "Swimmer" , "Small northern Pacific salmon" .)