Finch, outside science rooms, backing participant in rally? (6,4)
I believe the answer is:
tennis ball
'participant in rally?' is the definition.
(I've seen this before)
'finch outside science rooms backing' is the wordplay.
'finch' becomes 'linnet' (linnet is a kind of finch).
'outside' indicates putting letters inside (some letters go outside others).
'science rooms' becomes 'labs' (I've seen this in another clue).
'backing' says the letters should be written backwards.
'linnet' going around 'labs' is 'llabsinnet'.
'llabsinnet' written backwards gives 'TENNIS-BALL'.
(Other definitions for tennis ball that I've seen before include "Something hit by racquets" , "Item hit with rackets" , "Bouncer" , "Wimbledon equipment" , "Item for Wimbledon game" .)