Overshadow telecoms provider claiming high speeds (7)
I believe the answer is:
eclipse
'overshadow' is the definition.
(I know that eclipse is a more specific form of the action overshadow)
'telecoms provider claiming high speeds' is the wordplay.
'telecoms provider' becomes 'EE' (EE is a telecommunications company).
'claiming' indicates putting letters inside.
'high speeds' becomes 'clips' (clip can informally mean a speed or rate).
'ee' placed around 'clips' is 'ECLIPSE'.
(Other definitions for eclipse that I've seen before include "Outshine, throw into obscurity" , "Heavenly event; drake's time of dull plumage" , "Outdo" , "Darkening of the sun" , "The blotting out of a heavenly body" .)