Reckon Hoskins must open unopened premium Bud (8)
I believe the answer is:
estimate
'reckon' is the definition.
(estimating is a kind of reckoning)
'hoskins must open unopened premium bud' is the wordplay.
'hoskins' becomes 'I' (where the crossword setter is 'Hoskins').
'must open' is an insertion indicator (inserted letters open up a gap in the word).
'unopened premium' becomes 'est' (I can't explain this - if you can you should give a lot more credence to this answer).
'bud' becomes 'mate' (I've seen this in another clue).
'est'+'mate'='estmate'
'i' going into 'estmate' is 'ESTIMATE'.
Can you help me to learn more?
(Other definitions for estimate that I've seen before include "damage" , "Guess the cost" , "Quotation" , "Meatiest (anag) - figure" , "Calculate approximately" .)