Dominant peak has shortened (5)
I believe the answer is:
alpha
'dominant' is the definition.
(I've seen this before)
'peak has shortened' is the wordplay.
'peak' becomes 'alp' (I've seen this before).
'shortened' means to remove the last letter.
'has' with its last letter taken away is 'ha'.
'alp'+'ha'='ALPHA'
(Other definitions for alpha that I've seen before include "Dominant" , "First (class)" , "Top star" , "First-rate" , "A for Aristophanes!" .)