Dropper of two jacks, the second picked up? (8)
I believe the answer is:
abseiler
'dropper' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'two jacks the second picked up?' is the wordplay.
'two jacks' can mean two replacements for 'jack'.
'jack' becomes 'ab' (both can mean a sailor).
'jack' becomes 'seil' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the second' becomes 're' (prefix meaning second in eg review, re-education).
'picked up?' is a reversal indicator (I've seen 'pick up' mean this).
're' written backwards gives 'er'.
'ab'+'seil'+'er'='ABSEILER'
'of' is the link.
Can you help me to learn more?
(Other definitions for abseiler that I've seen before include "one on the way down" , "one clinging to cliff-face?" , "who has an on-line comedown" , "taking rope down" , "One descending using ropes" .)