Four unclued lights (eight words in all) form a phrase describing an activity. The other unclued lights give the author, and names of three participants
 

Across

 
9    Grow variable and impressionable (4)
10    Country in bad state, one further reduced (9, two words)
17    Watch acceleration in speed, say, when reversing (5)
18    Nothing bad about fruit (5)
20    In Venice perhaps time one takes a marine group (7)
21    Second day in twelve for one in pod (7)
23    Start to collapse with low sound, and fall to bits (7)
24    Caught by piece of chain in prison (5)
25    Beer-lovers see no point in one taking shots (5)
27   ...