Seaman holding all the cards? (8)
I believe the answer is:
deckhand
'seaman' is the definition.
(I know that deckhand is a type of seaman)
'all the cards?' is the wordplay.
'all' becomes 'deck' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the cards?' becomes 'hand' (I've seen this before).
'deck'+'hand'='DECKHAND'
'holding' acts as a link.
Can you help me to learn more?
(Other definitions for deckhand that I've seen before include "Ship worker" , "seaman" , "one employed on board?" , "crew member" .)