IM isn’t seen sat playing with a GM (7,7)

'im isn't' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'sat playing with a gm' is the wordplay.
I cannot really see how this works, but
an anagram of 'sat' is 'sta' which is within the answer.
'playing' could be 'in' and 'in' is found in the answer.
'a' is present in the answer.
'gm' could be 'g' (synonyms) and 'g' is found in the answer.
This explanation may well be incorrect...

'seen' acts as a link.

Can you help me to learn more?