Resolute, yet lacking firmness holding a tin-opener (7)
I believe the answer is:
doughty
'resolute' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for doughty that I've seen before include "Fearless" , "Brave, resolute (arch.)" , "Stout-hearted, valiant" .)