Turning up, bang on about leader of Rat Pack (5)
I believe the answer is:
tarot
'pack' is the definition.
(type of card pack)
'turning up bang on about leader of rat' is the wordplay.
'turning up' shows that the letters should be reversed in order.
'bang on' becomes 'to a T' (both can mean precisely or exactly right).
'about' means one lot of letters goes inside another.
'leader of' indicates taking the first letters.
The initial letter of 'rat' is 'r'.
'toat' enclosing 'r' is 'torat'.
'torat' in reverse letter order is 'TAROT'.
(Other definitions for tarot that I've seen before include "Fortune-teller's pack of cards" , "Picture card" , "Sort of cards" , "Fortune teller's seventy eight card pack" , "Cards used for fortune telling" .)