Part of pack to turn alternately into hearts and diamonds (5)
I believe the answer is:
hound
'part of pack' is the definition.
(as in a pack of hunting dogs)
'to turn alternately into hearts and diamonds' is the wordplay.
'alternately' indicates alternate letters.
'into' means one lot of letters goes inside another.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'and' is a charade indicator (letters next to each other).
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'to'+'turn'='toturn'
The alternate letters of 'toturn' are 'oun'.
'h'+'d'='hd'
'oun' placed inside 'hd' is 'HOUND'.
(Other definitions for hound that I've seen before include "Hunting dog; harass" , "despicable creature" , "unpleasant character" , "despicable person" , "badger" .)