Pack member’s flag (4)
I believe the answer is:
jack
I believe this is a double definition.
'pack member's' is the first definition.
(member of a pack of cards)
'flag' is the second definition.
(I know that jack is a type of flag)
(Other definitions for jack that I've seen before include "Name for sailor" , "Knave (cards)" , "-- Sprat; -- Horner" , "Lifting device - flag - boy's name - card - fish" , "See 5 Down" .)