It's a smashing game! (6)
I believe the answer is:
tennis
'game' is the definition.
(tennis is a game)
'it's a smashing' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'ennis' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tennis that I've seen before include "Roger Federer's game - anyone for it?" , "court contest" , "Henman's game" , "event" , "Game, orig. sphairistike" .)