The silent force of eg alt-rock? (8,4)
I believe the answer is:
keystone cops
'the silent force' is the definition.
(police force in a series of silent films)
'eg alt-rock?' is the wordplay.
I cannot really see how this works, but
'rock?' could be 'stone' ('stone' can be a synonym of 'rock') and 'stone' is located in the answer.
'eg alt' could be 'key' (Alt is a key on a computer keyboard) and 'key' is found in the answer.
The remaining letters 'cops' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' acts as a link.
(Another definition for keystone cops that I've seen is " constables in action".)