Last month, dire playing in tie-break (7)
I believe the answer is:
decider
'tie-break' is the definition.
I can't tell whether this defines the answer.
'last month dire playing in' is the wordplay.
'last month' becomes 'dec' (December, last month of the year).
'playing' indicates anagramming the letters.
'in' is an insertion indicator.
'dire' with letters rearranged gives 'ider'.
'dec' inserted within 'ider' is 'DECIDER'.
Can you help me to learn more?
(Other definitions for decider that I've seen before include "Settler" , "Clinching game" , "vital match" , "It produces a winner after a drawn series" , "It settles the matter" .)