Second set of cards in haversack (8)
I believe the answer is:
backpack
'haversack' is the definition.
(synonyms)
'second set of cards' is the wordplay.
'second' becomes 'back' (to second a motion is to back it).
'set of cards' becomes 'pack' (I've seen this before).
'back'+'pack'='BACKPACK'
'in' is the link.
(Other definitions for backpack that I've seen before include "Hike" , "travel light?" , "Knapsack" , "luggage" , "Rucksack" .)