Top seed unwise to sacrifice length with racket (7)
I believe the answer is:
noisily
'with racket' is the definition.
(in a way that makes a noise or racket)
'top seed unwise to sacrifice length' is the wordplay.
'top seed' becomes 'No I' (resembles No. 1, top seeded player).
'unwise' becomes 'silly'.
'to sacrifice' is a deletion indicator (some letters sacrificed).
'length' becomes 'l' (abbreviation in geometry).
'silly' with 'l' taken away is 'sily'.
'noi'+'sily'='NOISILY'
(Other definitions for noisily that I've seen before include "Rowing thus" , "With much loud sound" , "Unpleasantly loudly" , "making a din" , "in loud manner" .)