Admit high speed force used in breaking noses (7)
I believe the answer is:
confess
'admit' is the definition.
(confessing is a kind of admiting)
'high speed force used in breaking noses' is the wordplay.
'high speed' becomes 'c' (symbol for the speed of light).
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'used in' means one lot of letters goes inside another.
'breaking' is an anagram indicator.
'noses' anagrammed gives 'oness'.
'f' inserted into 'oness' is 'onfess'.
'c'+'onfess'='CONFESS'
(Other definitions for confess that I've seen before include "Concede" , "Admit (to wrongdoing)" , "Tell a priest one's sins" , "Admit (to crime)" , "Plead guilty (in order to receive absolution?)" .)