Seize unlicensed stock, unloading remaining packages first of all (5)
I believe the answer is:
'seize' is the definition.
(I know that seize can be written as usurp)
'unlicensed stock unloading remaining packages first of all' is the wordplay.
'first of all' suggests taking the first letters.
The initial letters of 'unlicensed stock unloading remaining packages' is 'USURP'.
(Other definitions for usurp that I've seen before include "Supplant (someone in power)" , "Dethrone" , "Seize possession of" , "Wrongfully assume" , "Take over occupation" .)