A performance enhancer: it might have a huge impact (8)
I believe the answer is:
asteroid
'it might have a huge impact' is the definition.
(as in an asteroid crashing into something)
'a performance enhancer' is the wordplay.
'performance enhancer' becomes 'steroid' (performance-enhancing drug).
'a'+'steroid'='ASTEROID'
(Other definitions for asteroid that I've seen before include "Any of the small planets revolving around the sun" , "Small planet orbiting the Sun" , "Bits of rock that move around the heavens" , "Ceres, say" , "Small celestial body moving around the sun" .)