Mastiff's tail wags around great group in pack (4,5)

'group in pack' is the definition.
(group in a pack of cards)

'mastiff's tail wags around great' is the subsidiary indication.
'tail' says to take the final letters.
'wags' becomes 'cards' ('card' can be a synonym of 'wag').
'around' indicates putting letters inside.
'great' becomes 'ace' (I've seen this before).
The final letter of 'mastiff' is 'f'.
'f'+'cards'='fcards'
'fcards' placed around 'ace' is 'facecards'.