Game for first of threes and nines st sixes and sevens (6)
I believe the answer is:
tennis
'game' is the definition.
(tennis is a game)
'first of threes and nines st sixes and sevens' is the wordplay.
I cannot quite see how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is located in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
The remaining letters 'tens' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Other definitions for tennis that I've seen before include "court contest" , "Roger Federer's game - anyone for it?" , "Real or lawn game" , "Evil trap set up for game" , "Sent in, strangely, for sport" .)