Brightest star's expression of surprise about record (5)
I believe the answer is:
alpha
'brightest star's' is the definition.
(I've seen this before)
'expression of surprise about record' is the wordplay.
'expression of surprise' becomes 'aha' (I've seen this before).
'about' indicates putting letters inside.
'record' becomes 'lp' (LP is an example).
'aha' placed around 'lp' is 'ALPHA'.
(Other definitions for alpha that I've seen before include "First in order of importance (for a Greek?)" , "starter" , "very good rating" , "A for Aristophanes!" , "First-rate" .)